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Document and Entity Information Document - USD ($)	12 Months Ended
Document and Entity Information Document - USD ($)	Dec. 31, 2017	Feb. 20, 2018	Jun. 30, 2017
Entity Information [Line Items]
Entity Registrant Name	WESCO INTERNATIONAL INC.
Entity Central Index Key	0000929008
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		47,056,716
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 2,738,884,142

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 117,953	$ 110,131
Trade accounts receivable, net of allowance for doubtful accounts of $19,309 and $17,242 in 2013 and 2012, respectively	1,170,080	1,034,402
Other accounts receivable	101,229	85,019
Inventories, net	956,148	821,441
Income taxes receivable	23,250	5,725
Prepaid expenses and other current assets	40,189	46,360
Total current assets	2,408,849	2,103,078
Property, buildings and equipment, net	156,445	157,607
Intangible assets, net	367,104	393,362
Goodwill	1,771,877	1,730,950
Deferred income taxes	24,203	15,803
Other assets	6,990	31,041
Total assets	4,735,468	4,431,841
Current Liabilities:
Accounts payable	799,520	684,721
Accrued payroll and benefit costs	72,686	49,250
Short-term debt	34,075	20,920
Current portion of long-term debt	1,224	1,218
Bank overdrafts	37,644	29,384
Accrued Income Taxes, Current	9,712	9,881
Other current liabilities	86,108	78,425
Total current liabilities	1,040,969	873,799
Long-term debt, net of discount of $174,686 and $183,644 in 2013 and 2012, respectively	1,313,261	1,363,135
Deferred income taxes	136,858	168,245
Other noncurrent liabilities	128,237	63,031
Total liabilities	2,619,325	2,468,210
Commitments and contingencies
Stockholders' Equity:
Preferred stock, $.01 par value; 20,000,000 shares authorized, no shares issued or outstanding	0	0
Additional capital	999,156	986,020
Retained earnings	2,079,697	1,914,757
Treasury stock, at cost; 18,179,275 and 18,102,528 shares in 2013 and 2012, respectively	(647,158)	(542,537)
Accumulated other comprehensive income	(312,590)	(391,971)
Total WESCO International stockholders' equity	2,116,143	1,963,631
Noncontrolling interest	(3,596)	(3,269)
Total stockholders' equity	2,119,739	1,966,900
Total liabilities and stockholders' equity	4,735,468	4,431,841
Common Stock
Stockholders' Equity:
Common Stock	591	588
Common Class B
Stockholders' Equity:
Common Stock	$ 43	$ 43

CONSOLIDATED BALANCE SHEETS CONSOLIDATED BALANCE SHEETS (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 21,313	$ 22,007
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 14,224	$ 17,278
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Treasury stock, shares outstanding	16,375,653	14,545,715
Common Stock
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	210,000,000	210,000,000
Common stock, shares issued	59,045,762	58,817,781
Common stock, shares outstanding	47,009,540	48,611,497
Common Class B
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	20,000,000	20,000,000
Common stock, shares issued	4,339,431	4,339,431
Common stock, shares outstanding	0	0

CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net sales	$ 7,679,021	$ 7,336,017	$ 7,518,487
Cost of goods sold (excluding depreciation and amortization below)	6,194,366	5,887,814	6,024,826
Selling, general and administrative expense	1,099,748	1,049,286	1,054,951
Depreciation and amortization	64,017	66,858	64,968
Income from operations	320,890	332,059	373,742
Interest expense, net	68,450	76,575	69,832
Gain (Loss) on Extinguishment of Debt	0	(123,933)	0
Income before income taxes	252,440	131,551	303,910
Provision for income taxes	89,307	30,431	95,537
Net income	163,133	101,120	208,373
Net loss attributable to noncontrolling interest	(327)	(468)	(2,314)
Net Income attributable to WESCO International, Inc.	163,460	101,588	210,687
Comprehensive Income:
Foreign currency translation adjustment	85,762	38,275	(225,795)
Benefit plan adjustments	(6,381)	(2,485)	4,532
Comprehensive income attributable to WESCO International, Inc.	$ 242,841	$ 137,378	$ (10,576)
Earnings per share :
Basic	$ 3.42	$ 2.30	$ 4.85
Diluted	$ 3.38	$ 2.10	$ 4.18

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Additional Capital	Common Class B	Common Stock
Common Stock, Value, Issued			$ 43	$ 588
Common Stock, Shares, Issued			4,339,431	58,817,781
Additional Paid in Capital, Common Stock	$ 986,020
Retained Earnings (Accumulated Deficit)	1,914,757
Treasury Stock, Value	$ (542,537)
Treasury Stock, Shares	(14,545,715)
Stockholders' Equity Attributable to Noncontrolling Interest	$ (3,269)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(391,971)
Treasury Stock, Value, Acquired, Par Value Method		$ 38
Treasury Stock, Value, Acquired, Cost Method	$ (100,038)
Treasury Stock, Shares, Acquired	(1,778,537)
Net Income (Loss) Attributable to Noncontrolling Interest	$ 327
Benefit plan adjustments	6,381
Stockholders' equity at period end at Dec. 31, 2017	2,116,143
Net Income attributable to WESCO International, Inc.	163,460
Common Stock, Value, Issued			$ 43	$ 591
Common Stock, Shares, Issued			4,339,431	59,045,762
Additional Paid in Capital, Common Stock	999,156
Retained Earnings (Accumulated Deficit)	2,079,697
Treasury Stock, Value	$ (647,158)
Treasury Stock, Shares	(16,375,653)
Stockholders' Equity Attributable to Noncontrolling Interest	$ (3,596)
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (312,590)

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' Equity Parenthetical [Abstract]
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options	$ 0	$ 67	$ 1,403
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	$ (2,361)	$ (302)	$ 1,661

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities:
Net income	$ 163,133	$ 101,120	$ 208,373
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	64,017	66,858	64,968
Stock-based compensation expense	14,809	12,493	12,899
Amortization of Debt Issuance Costs and Discounts	3,984	6,684	12,195
Gain (Loss) on Extinguishment of Debt	0	123,933	0
(Gain) loss on sale of property, buildings and equipment	(4,038)	(4,702)	(45)
Other Operating Activities, Cash Flow Statement	(79)	836	11,627
Interest related to uncertain tax positions	100	1,200	8,700
Deferred income taxes	(50,396)	(45,174)	42,850
Changes in assets and liabilities
Increase (Decrease) in Accounts Receivable	(112,977)	56,767	40,102
Other accounts receivable	(14,163)	(1,628)	57,242
Inventories, net	(119,002)	(1,612)	2,410
Prepaid expenses and other current assets	11,334	13,207	(8,517)
Accounts payable	(102,870)	40,607	55,914
Accrued payroll and benefit costs	24,679	(1,922)	(15,015)
Other current and noncurrent liabilities	64,793	15,654	(66,872)
Net cash provided by operating activities	149,122	300,235	283,049
Investing Activities:
Capital expenditures	(21,507)	(17,957)	(21,658)
Acquisition payments, net of cash acquired	0	(50,890)	(151,595)
Proceeds from sale of assets	6,766	8,361	3,023
Payments for (Proceeds from) Other Investing Activities	9,446	(10,000)	0
Net cash used in investing activities	(5,295)	(70,486)	(170,230)
Financing Activities:
Proceeds from issuance of short-term debt	175,819	111,458	102,033
Repayments of short-term debt	(164,030)	(131,501)	(101,353)
Proceeds from issuance of long-term debt	1,504,636	2,082,738	1,528,578
Repayments of long-term debt	(1,556,636)	(2,323,568)	(1,435,820)
Debt issuance costs	(915)	(6,002)	(3,359)
Repurchase of common stock	(106,792)	(4,818)	(155,805)
Proceeds from (Payments for) Other Financing Activities	6,722	(4,570)	(2,089)
Net cash provided (used) by financing activities	(141,196)	(276,263)	(67,815)
Effect of exchange rate on cash and cash equivalents	5,191	(3,634)	(13,044)
Net change in cash and cash equivalents	7,822	(50,148)	31,960
Cash and cash equivalents at the beginning of period	110,131	160,279	128,319
Cash and cash equivalents at the end of period	117,953	110,131	160,279
Supplemental disclosures:
Cash paid for interest	63,795	74,391	66,342
Cash paid for taxes	65,117	76,293	74,213
Non-cash investing and financing activities:
Payments to Acquire Equipment on Lease	$ 552	$ 1,143	$ 288

ORGANIZATION	12 Months Ended
ORGANIZATION	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION	ORGANIZATION WESCO International, Inc. ("WESCO International") and its subsidiaries (collectively, “WESCO” or the "Company"), headquartered in Pittsburgh, Pennsylvania, is a full-line distributor of electrical, industrial and communications maintenance, repair and operating ("MRO") and original equipment manufacturer ("OEM") products, construction materials, and advanced supply chain management and logistics services used primarily in the industrial, construction, utility and commercial, institutional and government markets. WESCO serves approximately 70,000 active customers globally, through approximately 500 branches and 10 distribution centers located primarily in the United States, Canada and Mexico, with operations in 15 additional countries.

ACCOUNTING POLICIES

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Significant Accounting Policies [Text Block]

2. ACCOUNTING POLICIES

Basis of Consolidation

The consolidated financial statements include the accounts of WESCO International and all of its subsidiaries. All significant intercompany accounts and transactions have been eliminated in consolidation.

Revision of Prior Period Financial Statements

In the third quarter of 2017, management determined that the Company's income taxes receivable and payable and other tax account balances were overstated as of December 31, 2016 by a cumulative net amount of $46.4 million, which related to multiple prior periods. The Company also identified a $10.2 million understatement related to deferred income taxes and goodwill. In accordance with Staff Accounting Bulletin (“SAB”) No. 99, Materiality, and SAB No. 108, Considering the Effects of Prior Year Misstatements when Quantifying Misstatements in Current Year Financial Statements, management concluded that these misstatements are not material to the Company's previously issued annual and interim financial statements. Correcting the effected financial statement line items in the year ended December 31, 2017 would have materially misstated the consolidated financial statements presented herein. Accordingly, the Consolidated Balance Sheet at December 31, 2016 and the Consolidated Statements of Stockholders' Equity for the years ended December 31, 2016 and 2015 have been revised in this Annual Report on Form 10-K. There was an immaterial effect on the Consolidated Statements of Income and Comprehensive Income (Loss) for the years ended December 31, 2016 and 2015, and no effect on the Consolidated Statements of Cash Flows for such periods.

The following table presents the effects on the financial statement line items that were revised:



	December 31, 2016
	As Reported			Adjustment			As Revised
	(In thousands)
Income taxes receivable	$	72,881		$	(67,156	)	$	5,725
Prepaid expenses and other current assets	48,583			(2,223		)	46,360
Total current assets	2,172,457			(69,379		)	2,103,078
Goodwill	1,720,714			10,236			1,730,950
Total assets	4,490,984			(59,143		)	4,431,841
Income taxes payable	32,879			(22,998		)	9,881
Total current liabilities	896,797			(22,998		)	873,799
Deferred income taxes	158,009			10,236			168,245
Total liabilities	2,480,972			(12,762		)	2,468,210
Retained earnings ⁽¹⁾	1,956,532			(41,775		)	1,914,757
Accumulated other comprehensive loss ⁽¹⁾	(387,365		)	(4,606		)	(391,971		)
Total WESCO International, Inc. stockholders' equity	2,013,281			(46,381		)	1,966,900
Total stockholders' equity	2,010,012			(46,381		)	1,963,631
Total liabilities and stockholders' equity	4,490,984			(59,143		)	4,431,841


⁽¹⁾	These financial statement line items have been revised as of December 31, 2015 and 2014 in the Consolidated Statements of Stockholders' Equity for the years ended December 31, 2017, 2016 and 2015.

Use of Estimates

The preparation of financial statements in conformity with generally accepted accounting principles in the United States of America requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Although these estimates are based on management’s best knowledge of current events and actions WESCO may undertake in the future, actual results may ultimately differ from the estimates.

Revenue Recognition

Revenues are recognized for product sales when title, ownership and risk of loss pass to the customer or for services when the service is rendered. In the case of stock sales and special orders, a sale occurs at the time of shipment from WESCO's distribution point, as the terms of WESCO’s sales are typically FOB shipping point. In cases where WESCO processes customer orders that ship directly from suppliers, revenue is recognized once the product is shipped and title has passed. In all cases, revenue is recognized once the sales price to the customer is fixed or is determinable and WESCO has reasonable assurance as to the collectability.

WESCO provides integrated supply services to certain customers, which include some or all of the following: determine inventory stocking levels; establish inventory reorder points; launch purchase orders; receive material; pack away material; and, pick material for order fulfillment. WESCO recognizes revenue for these services in the period rendered based upon a previously negotiated fee arrangement. WESCO also sells inventory to these customers and recognizes revenue at the time title and risk of loss transfers to the customer. The amount of revenue recognized for integrated supply services totaled $26.2 million, $27.1 million, and $35.1 million in 2017, 2016 and 2015, respectively.

Selling, General and Administrative Expenses

WESCO includes warehousing, purchasing, branch operations, information services, and marketing and selling expenses in this category, as well as other types of general and administrative costs.

Supplier Volume Rebates

WESCO receives volume rebates from certain suppliers based on contractual arrangements with such suppliers. Volume rebates are included within other accounts receivable in the Consolidated Balance Sheets, and represent the estimated amounts due to WESCO under the rebate provisions of the various supplier contracts. The corresponding rebate income is derived from the level of actual purchases made by WESCO and is recorded as a reduction to cost of goods sold. Receivables under the supplier rebate program were $72.7 million at December 31, 2017 and $64.2 million at December 31, 2016. Supplier volume rebate rates have historically ranged between approximately 0.9% and 1.4% of sales depending on market conditions. In 2017, the rebate rate was 1.3%.

Shipping and Handling Costs and Fees

WESCO records the costs and fees associated with transporting its products to customers as a component of selling, general and administrative expenses. These costs totaled $61.8 million, $57.9 million and $59.4 million in 2017, 2016 and 2015, respectively.

Cash Equivalents

Cash equivalents are defined as highly liquid investments with original maturities of 90 days or less when purchased.

Asset Securitization

WESCO maintains control of the receivables transferred pursuant to its accounts receivable securitization program (the “Receivables Facility”); therefore, the transfers do not qualify for “sale” treatment. As a result, the transferred receivables remain on the balance sheet, and WESCO recognizes the related secured borrowing. The expenses associated with the Receivables Facility are reported as interest expense in the Consolidated Statements of Income and Comprehensive Income (Loss).

Allowance for Doubtful Accounts

WESCO maintains allowances for doubtful accounts for estimated losses resulting from the inability of its customers to make required payments. WESCO has a systematic procedure using estimates based on historical data and reasonable assumptions of collectability made at the local branch level and on a consolidated corporate basis to calculate the allowance for doubtful accounts. If the financial condition of WESCO’s customers were to deteriorate, resulting in an impairment of their ability to make payments, additional allowances may be required. The allowance for doubtful accounts was $21.3 million at December 31, 2017 and $22.0 million at December 31, 2016. The total amount recorded as selling, general and administrative expense related to bad debts was $8.5 million, $5.9 million and $6.1 million for 2017, 2016 and 2015, respectively.

Inventories

Inventories primarily consist of merchandise purchased for resale and are stated at the lower of cost and net realizable value. Cost is determined principally under the average cost method. WESCO makes provisions for obsolete or slow-moving inventories as necessary to reflect reductions in value. WESCO writes down its inventories to net realizable value based on internal factors derived from historical analysis of actual losses. On a retrospective basis, WESCO identifies items in excess of 36 months supply relative to demand or movement. WESCO then analyzes the ultimate disposition of identified excess inventories as they are sold, returned to supplier, or scrapped. This historical item-by-item analysis allows WESCO to develop an estimate of the likelihood that an item identified as being in excess supply ultimately becomes obsolete. WESCO applies the estimate to inventories currently in excess of 36 months supply, and reduces the carrying value of its inventories by the derived amount. Reserves for excess and obsolete inventories were $28.6 million and $27.3 million at December 31, 2017 and 2016, respectively. The total expense related to excess and obsolete inventories, included in cost of goods sold, was $8.8 million, $7.3 million and $8.6 million for 2017, 2016 and 2015, respectively. WESCO absorbs into the cost of inventories certain overhead expenses such as purchasing, receiving and storage and at December 31, 2017 and 2016, $70.7 million and $65.3 million, respectively, of these costs were included in ending inventories.

Property, Buildings and Equipment

Property, buildings and equipment are recorded at cost. Depreciation expense is determined using the straight-line method over the estimated useful lives of the assets. Leasehold improvements are amortized over either their respective lease terms or their estimated lives, whichever is shorter. Estimated useful lives range from five to forty years for buildings and leasehold improvements and three to ten years for furniture, fixtures and equipment.

Capitalized computer software costs are amortized using the straight-line method over the estimated useful life, typically three to five years, and are reported at the lower of unamortized cost or net realizable value.

Expenditures for new facilities and improvements that extend the useful life of an asset are capitalized. Ordinary repairs and maintenance are expensed as incurred. When property is retired or otherwise disposed of, the cost and the related accumulated depreciation are removed from the accounts and any resulting gains or losses are recorded and reported as selling, general and administrative expenses.

Of WESCO’s $156.4 million net book value of property, buildings and equipment as of December 31, 2017, $97.6 million consists of land, buildings and leasehold improvements and are geographically dispersed among WESCO’s 500 branches and 10 distribution centers, mitigating the risk of impairment. WESCO assesses its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of any such assets may not be fully recoverable. Changes in circumstances include technological advances, changes in the business model, capital structure, economic conditions or operating performance. The evaluation is based upon, among other things, utilization, serviceability and assumptions about the estimated future undiscounted cash flows that these assets are expected to generate. When the sum of the undiscounted cash flows is less than the carrying value of the asset or asset group, an impairment loss is recognized to the extent that carrying value exceeds fair value. Management applies its best judgment when performing these evaluations.

Goodwill and Indefinite-Lived Intangible Assets

Goodwill and indefinite-lived intangible assets are tested for impairment annually during the fourth quarter using information available at the end of September, or more frequently if triggering events occur, indicating that their carrying value may not be recoverable. WESCO tests for goodwill impairment on a reporting unit level and the evaluation involves comparing the fair value of each reporting unit to its carrying value. The fair values of the reporting units are determined using a combination of a discounted cash flow analysis and market multiples. Assumptions used for these fair value techniques are based on a combination of historical results, current forecasts, market data and recent economic events. WESCO evaluates the recoverability of indefinite-lived intangible assets using the relief-from-royalty method based on projected financial information. At December 31, 2017 and 2016, respectively, goodwill and indefinite-lived trademarks totaled $1.87 billion and $1.83 billion.

We performed our annual impairment testing of goodwill and indefinite-lived intangible assets during the fourth quarter. A possible indicator of goodwill impairment is the relationship of a company’s market capitalization to its book value. As of December 31, 2017, our market capitalization exceeded our book value and the fair values of our reporting units exceeded their carrying values. Accordingly, there were no impairment losses identified as a result of our annual test.

The determination of fair value involves significant management judgment and management applies its best judgment when assessing the reasonableness of financial projections. Fair values are sensitive to changes in underlying assumptions and factors. As a result, there can be no assurance that the estimates and assumptions made for purposes of the annual goodwill and indefinite-lived intangible impairment tests will prove to be an accurate prediction of future results.

Definite Lived Intangible Assets

Intangible assets are amortized over 2 to 20 years. A portion of intangible assets related to certain customer relationships are amortized using an accelerated method whereas all other intangible assets subject to amortization use a straight-line method that reflects the pattern in which the economic benefits of the respective assets are consumed or otherwise used. Intangible assets are tested for impairment if events or circumstances occur indicating that the respective asset might be impaired.

Insurance Programs

WESCO uses commercial insurance for auto, workers’ compensation, casualty and health claims, and information technology as a risk-reduction strategy to minimize catastrophic losses. The Company’s strategy involves large deductible policies where WESCO must pay all costs up to the deductible amount. WESCO estimates the reserve for these programs based on historical incident rates and costs. The assumptions included in developing this accrual include the period of time between the incurrence and payment of a claim. The total liability related to the insurance programs was $13.9 million and $9.5 million at December 31, 2017 and 2016, respectively.

Income Taxes

WESCO accounts for income taxes under the asset and liability method, which requires the recognition of deferred income taxes for events that have future tax consequences. Under this method, deferred income taxes are recognized (using enacted tax laws and rates) based on the future income tax effects of differences in the carrying amounts of assets and liabilities for financial reporting and tax purposes. The effect of a tax rate change on deferred tax assets and liabilities is recognized in income in the period of change.

WESCO recognizes deferred tax assets at amounts that are expected to be realized. To make such determination, management evaluates all positive and negative evidence, including but not limited to, prior, current and future taxable income, tax planning strategies and future reversals of existing temporary differences. A valuation allowance is recognized if it is “more-likely-than-not” that some or all of a deferred tax asset will not be realized. WESCO regularly assesses the realizability of deferred tax assets.

WESCO accounts for uncertainty in income taxes using a "more-likely-than-not" recognition threshold. Due to the subjectivity inherent in the evaluation of uncertain tax positions, the tax benefit ultimately recognized may materially differ from the estimate. WESCO recognizes interest and penalties related to uncertain tax benefits as part of interest expense and income tax expense, respectively.

The Tax Cuts and Jobs Act of 2017 (the “TCJA”) imposes a one-time tax on the deemed repatriation of undistributed foreign earnings. Notwithstanding the effects of applying such provisions of the TCJA, WESCO continues to assert that the earnings of its foreign subsidiaries are indefinitely reinvested. However, as a result of the TCJA, the Company is reevaluating its intent and ability to repatriate foreign cash based upon the available liquidity and cash flow needs of its foreign subsidiaries and will disclose in future filings any change in its intention to repatriate undistributed foreign earnings and any resulting income tax impacts. Until the Company completes this reevaluation, it is not practicable to determine the amount of any unrecognized deferred income taxes on these undistributed foreign earnings.

The provisions of the TCJA also introduce U.S. taxation on certain global intangible low-taxed income ("GILTI"). WESCO has elected to account for any GILTI tax that arises in future periods as a component of income tax expense.

Provisional amounts are recorded for certain income tax effects of the TCJA for which the accounting is incomplete, but a reasonable estimate can be determined. Provisional amounts, or adjustments to provisional amounts, identified during the period ending on or before one year from the TCJA's enactment date are recognized as an adjustment to income tax expense or benefit from continuing operations in the period the amounts are determined.

Debt Issuance Costs

WESCO capitalizes costs associated with the issuance of debt and such costs are amortized over the term of the respective debt instrument on a straight-line basis. Debt issuance costs are presented in the Consolidated Balance Sheets as a direct reduction from the carrying amount of the related debt liability. Upon prepayment of debt, the Company accelerates the recognition of an appropriate amount of the costs as refinancing or extinguishment of debt. During the year ended December 31, 2017, the Company capitalized debt issuance costs of $0.9 million. As of December 31, 2017 and 2016, unamortized debt issuance costs of $13.7 million and $16.5 million were recorded in the Consolidated Balance Sheets, respectively.

Convertible Debentures

WESCO separately accounted for the liability and equity components of the 6.0% Convertible Senior Debentures due 2029 (the "2029 Debentures") in a manner that reflected its non-convertible debt borrowing rate. WESCO estimated its non-convertible debt borrowing rate through a combination of discussions with its financial institutions and review of relevant market data. The discounts to the convertible debt balances were amortized to interest expense, using the effective interest method, over the implicit life of the debentures.

Foreign Currency

The local currency is the functional currency for the majority of WESCO’s operations outside the United States. Assets and liabilities of these operations are translated to U.S. dollars at the exchange rate in effect at the end of each period. Income statement accounts are translated at an exchange rate that approximates the average for the period. Translation adjustments arising from the use of differing exchange rates from period to period are included as a component of other comprehensive income (loss) within stockholders’ equity. Gains and losses from foreign currency transactions are included in net income for the period.

Defined Benefit Pension Plan

Liabilities and expenses for pension benefits are determined using actuarial methodologies and incorporate significant assumptions, including the interest rate used to discount the future estimated cash flows, the expected long-term rate of return on plan assets, and several assumptions relating to the employee workforce (salary increases, retirement age, and mortality).

The interest rate used to discount future estimated cash flows is determined using the Canadian Institute of Actuaries ("CIA") methodology, which references yield curve information provided by Fiera Capital and matches expected benefit payments. The expected long-term rate of return on plan assets is applied to the fair market-related value of plan assets. The discount rate used to determine the projected benefit obligations for the Canadian pensions was 3.5% at December 31, 2017.

Stock-Based Compensation

WESCO's stock-based employee compensation plans are comprised of stock-settled stock appreciation rights, restricted stock units, and performance-based awards. Compensation cost for all stock-based awards is measured at fair value on the date of grant, and compensation cost is recognized, net of forfeitures, over the service period for awards expected to vest. The fair value of stock-settled appreciation rights and performance-based awards with market conditions is determined using the Black-Scholes and Monte Carlo simulation models, respectively. The fair value of restricted stock units with service conditions and performance-based awards with performance conditions is determined by the grant-date closing price of WESCO's common stock. Expected volatilities are based on historical volatility of WESCO's common stock. WESCO estimates the expected life of stock-settled stock appreciation rights using historical data pertaining to option exercises and employee terminations. The risk-free rate is based on the U.S. Treasury yields in effect at the time of grant. The forfeiture assumption is based on WESCO's historical employee behavior, which is reviewed on an annual basis. No dividends are assumed for stock-based awards. For stock appreciation rights that are exercised and for restricted stock units and performance-based award that vest, shares are issued out of WESCO's outstanding common stock.

Treasury Stock

Common stock purchased for treasury is recorded at cost. At the date of subsequent reissue, the treasury stock account is reduced by the cost of such stock, with cost determined on a weighted-average basis.

Fair Value of Financial Instruments

The Company measures the fair value of financial assets and liabilities in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) 820, "Fair Value Measurements and Disclosures." ASC 820 establishes a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value and requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to measurements involving significant unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are as follows:


•	Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities that are accessible at the measurement date.

•

Level 2 inputs include inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices in active markets for similar assets and liabilities, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of assets or liabilities.


•	Level 3 inputs are unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions.

The Company’s financial instruments consist of cash and cash equivalents, accounts receivable, accounts payable, bank overdrafts, other accrued liabilities, and outstanding indebtedness. The reported carrying amounts of WESCO’s debt instruments totaled $1.36 billion and $1.40 billion at December 31, 2017 and 2016, respectively, and approximated their fair values which totaled $1.39 billion and $1.42 billion, respectively. The Company uses a market approach to fair value all of its debt instruments, utilizing quoted prices in active markets, interest rates and other relevant information generated by market transactions involving similar instruments. Therefore, all of the Company's debt instruments are classified as Level 2 within the valuation hierarchy. For all of the Company's remaining financial instruments, carrying values are considered to approximate fair value.

Environmental Expenditures

WESCO has facilities and operations that distribute certain products that must comply with environmental regulations and laws. Expenditures for current operations are expensed or capitalized, as appropriate. Expenditures relating to existing conditions caused by past operations, and that do not contribute to future revenue, are expensed. Liabilities are recorded when remedial efforts are probable and the costs can be reasonably estimated.

Recently Adopted Accounting Pronouncements

In March 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, which simplifies several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The amendments in this ASU affect all entities that issue share-based payment awards to their employees. The Company adopted this ASU in the first quarter of 2017. The amendment related to the recognition of excess tax benefits and deficiencies was applied prospectively and lowered the Company's effective tax rate by approximately 1% for the year ended December 31, 2017. The amendment related to the presentation of excess tax benefits on the statement of cash flows was also applied prospectively, and did not have a material impact on WESCO's cash flows. The other amendments, which were adopted by the Company according to the respective transition requirements, had no impact on the consolidated financial statements and notes thereto.

In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory. The purpose of ASU 2016-16 is to simplify the income tax accounting of an intra-entity transfer of an asset other than inventory and to record its effect when the transfer occurs. The Company early adopted this ASU on a modified retrospective basis in the first quarter of 2017. The adoption of this ASU did not have a material impact on WESCO's financial position and it had no impact on its results of operations or cash flows.

Recently Issued Accounting Pronouncements

In August 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers: Deferral of Effective Date. The Company previously reported that in May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers, which provides a framework for addressing revenue recognition issues and replaces almost all existing revenue recognition guidance in current U.S. generally accepted accounting principles. The core principle of ASU 2014-09 is for companies to recognize revenue for the transfer of goods or services to customers in amounts that reflect the consideration to which the company expects to be entitled in exchange for those goods or services. ASU 2014-09 will also result in enhanced disclosures about revenue, provide guidance for transactions that were not previously addressed comprehensively, and improve guidance for multiple-element arrangements. The amendments in ASU 2015-14 defer the effective date of the new revenue recognition guidance to annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. During 2016, the FASB issued four ASUs that address implementation issues and correct or improve certain aspects of the new revenue recognition guidance, including ASU 2016-08, Principal versus Agent Considerations (Reporting Revenue Gross versus Net), ASU 2016-10, Identifying Performance Obligations and Licensing, ASU 2016-12, Narrow-Scope Improvements and Practical Expedients and ASU 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers. These ASUs do not change the core principles in the revenue recognition standard outlined above. The Company developed a multiphase plan and established a cross-functional team to evaluate and implement the new standard. Management completed the diagnostic and testing phase of the project, which involved reviewing various customer contracts and comparing current accounting to the requirements of the new standard. Currently management is evaluating the new disclosure requirements and identifying and implementing appropriate changes to the Company's business processes and controls to support recognition and disclosure under the new standard. The new standard will be adopted in the first quarter of 2018 using the modified retrospective method. Except for new disclosures requirements, the adoption of this pronouncement is not expected to have a material impact on WESCO's consolidated financial statements.

In February 2016, the FASB issued ASU 2016-02, Leases, a comprehensive new standard that amends various aspects of existing accounting guidance for leases, including the recognition of a right-of-use asset and a lease liability in the balance sheet and disclosing key information about leasing arrangements. This guidance is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The new leasing standard requires modified retrospective transition, which requires application of the new guidance at the beginning of the earliest comparative period presented in the year of adoption. Management is currently evaluating the impact of this standard and right-of-use assets and lease liabilities will be recorded in the Consolidated Balance Sheets upon adoption. An estimate of the impact of this standard is not currently determinable.

In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which introduces new guidance for the accounting for credit losses on certain financial instruments. The amendments in ASU 2016-13 are effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years and early adoption is permitted. The Company does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto.

In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force). This ASU provides guidance on eight specific cash flow issues where there is diversity in practice. The amendments in this ASU are effective for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. Except for potential reclassifications within the statement of cash flows, the Company does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto.

In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment, which eliminates Step 2 from the goodwill impairment test. Under the amendments in this update, an entity should perform its annual, or interim, goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. An entity should apply the amendments in this ASU on a prospective basis. This guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. The Company does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto.

In March 2017, the FASB issued ASU 2017-07, Compensation—Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. Presently, net benefit cost is reported as an employee cost within operating income (or capitalized into assets when appropriate). This amendment requires the bifurcation of net benefit cost. The service component will be presented with other employee compensation costs in operating income (or capitalized in assets). The other components will be reported separately outside of operations, and will not be eligible for capitalization. This guidance is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The Company does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto.

In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting, which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The guidance is effective prospectively for all companies for annual periods beginning on or after December 15, 2017. Early adoption is permitted. The Company does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto.

Other pronouncements issued by the FASB or other authoritative accounting standards groups with future effective dates are either not applicable or are not expected to be significant to WESCO’s financial position, results of operations or cash flows.

GOODWILL AND INTANGIBLE ASSETS

12 Months Ended

Dec. 31, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

GOODWILL AND INTANGIBLE ASSETS

3. GOODWILL AND INTANGIBLE ASSETS

Goodwill

The following table sets forth the changes in the carrying value of goodwill:



	Year Ended December 31,
	2017		2016
	(In thousands)
Beginning balance January 1	$	1,730,950	$	1,681,662
Foreign currency exchange rate changes	40,927		21,434
Adjustments to goodwill for acquisitions ⁽¹⁾	—		17,618
Prior period revision (Note 2)	—		10,236
Ending balance December 31	$	1,771,877	$	1,730,950


⁽¹⁾	For the year ended December 31, 2016, adjustments relate to goodwill resulting from the preliminary allocation of the purchase price for Atlanta Electrical Distributors, LLC to the respective assets acquired and liabilities assumed, partially offset by an adjustment to goodwill related to deferred income taxes.

Intangible Assets

The components of intangible assets are as follows:



		December 31, 2017							December 31, 2016
	Life	Gross Carrying Amount ⁽¹⁾		Accumulated Amortization ⁽¹⁾			Net Carrying Amount		Gross Carrying Amount ⁽¹⁾		Accumulated Amortization ⁽¹⁾			Net Carrying Amount
		(In thousands)
Intangible assets:
Trademarks	Indefinite	$	100,249	$	—		$	100,249	$	96,962	$	—		$	96,962
Trademarks	4-15	25,118		(5,516		)	19,602		25,098		(3,426		)	21,672
Non-compete agreements	2-7	196		(102		)	94		196		(63		)	133
Customer relationships	2-20	377,270		(161,711		)	215,559		362,637		(126,835		)	235,802
Distribution agreements	10-19	39,515		(22,200		)	17,315		38,972		(19,295		)	19,677
Patents	10	48,310		(34,025		)	14,285		48,310		(29,194		)	19,116
		$	590,658	$	(223,554	)	$	367,104	$	572,175	$	(178,813	)	$	393,362


⁽¹⁾	Excludes the original cost and accumulated amortization of fully-amortized intangible assets.

Amortization expense related to intangible assets totaled $37.8 million, $39.1 million and $36.9 million for the years ended December 31, 2017, 2016 and 2015, respectively.

The following table sets forth the estimated amortization expense for intangible assets for the next five years:



For the year ending December 31,	(In thousands)
2018	$	37,848
2019	36,549
2020	34,621
2021	26,930
2022	24,350

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

CONCENTRATIONS OF CREDIT RISK AND SIGNIFICANT SUPPLIERS	12 Months Ended
CONCENTRATIONS OF CREDIT RISK AND SIGNIFICANT SUPPLIERS	Dec. 31, 2017
Risks and Uncertainties [Abstract]
CONCENTRATIONS OF CREDIT RISK AND SIGNIFICANT SUPPLIERS	4. CONCENTRATIONS OF CREDIT RISK AND SIGNIFICANT SUPPLIERS WESCO distributes its products and services and extends credit to a large number of customers in the industrial, construction, utility, and commercial, institutional and government markets. Based upon WESCO’s broad customer base, the Company has concluded that it has no material credit risk as a result of customer concentration. WESCO's largest supplier is Eaton Corporation, accounting for approximately 11% of its purchases in 2017, 2016 and 2015. Therefore, WESCO could potentially incur risk due to supplier concentration.

ACQUISITIONS

12 Months Ended

Dec. 31, 2017

Business Combinations [Abstract]

ACQUISITIONS

5. ACQUISITIONS

The following table sets forth the consideration paid for acquisitions:



	Year Ended December 31,
	2017		2016			2015
	(In thousands)
Fair value of assets acquired	$	—	$	76,980		$	192,099
Fair value of liabilities assumed	—		25,058			39,836
Cash paid for acquisitions	$	—	$	51,922		$	152,263
Supplemental cash flow disclosure related to acquisitions:
Cash paid for acquisitions	$	—	$	51,922		$	152,263
Less: cash acquired	—		(1,032		)	(668		)
Cash paid for acquisitions, net of cash acquired	$	—	$	50,890		$	151,595

Atlanta Electrical Distributors, LLC

On March 14, 2016, WESCO Distribution, Inc. ("WESCO Distribution") completed the acquisition of Atlanta Electrical Distributors, LLC, an Atlanta-based electrical distributor focused on the construction and MRO markets from five locations in Georgia with approximately $85 million in annual sales. WESCO Distribution funded the purchase price paid at closing with borrowings under its revolving credit facility. The purchase price was allocated to the respective assets and liabilities based upon their estimated fair values as of the acquisition date. In addition to the cash paid at closing, the purchase price included a contingent payment that may be earned upon the achievement of certain financial performance targets over three consecutive one year periods. The fair value of the contingent consideration was determined using a probability-weighted outcome analysis and Level 3 inputs such as internal forecasts. This amount was initially accrued at the maximum potential payout under the terms of the purchase agreement and was reduced in 2017 to reflect a payout that is aligned with current financial performance. The fair value of intangibles was estimated by management and the allocation resulted in intangible assets of $21.8 million and goodwill of $30.0 million. The intangible assets include customer relationships of $15.8 million amortized over 13 and 14 years, a trademark of $6.0 million amortized over 13 years, and non-compete agreements of less than $0.1 million amortized over 5 years. No residual value was estimated for the intangible assets being amortized. The majority of goodwill is deductible for tax purposes.

Hill Country Electric Supply, LP and Needham Electric Supply Corporation

On May 1, 2015, WESCO Distribution completed the acquisition of Hill Country Electric Supply, LP ("Hill Country"), an electrical distributor focused on the commercial construction market from nine locations in Central and South Texas with approximately $140 million in annual sales. WESCO Distribution funded the purchase price paid at closing with borrowings under its prior revolving credit facility. The purchase price was allocated to the respective assets and liabilities based upon their estimated fair values as of the acquisition date. The fair value of intangibles was estimated by management and the allocation resulted in intangible assets of $21.1 million and goodwill of $16.2 million. The majority of goodwill is deductible for tax purposes.

On October 30, 2015, WESCO Distribution completed the acquisition of Needham Electric Supply Corporation ("Needham"), an electrical distributor focused on the commercial construction and lighting national account markets from 24 locations in Massachusetts, New Hampshire and Vermont with approximately $115 million in annual sales. WESCO Distribution funded the purchase price paid at closing with cash and borrowings under its revolving credit facility. The purchase price was allocated to the respective assets and liabilities based upon their estimated fair values as of the acquisition date. The fair value of intangibles was estimated by management and the allocation resulted in intangible assets of $31.0 million and goodwill of $35.7 million. The majority of goodwill is deductible for tax purposes.

For the acquisitions of Hill Country and Needham that were made in 2015, the intangible assets include customer relationships of $37.6 million amortized over 11 to 14 years, trademarks of $14.3 million amortized over 12 and 13 years, and other intangibles of $0.2 million. No residual value is estimated for the intangible assets being amortized.

PROPERTY, BUILDINGS AND EQUIPMENT

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

PROPERTY, BUILDINGS AND EQUIPMENT

6. PROPERTY, BUILDINGS AND EQUIPMENT

The following table sets forth the components of property, buildings and equipment:



	As of December 31,
	2017			2016
	(In thousands)
Buildings and leasehold improvements	$	117,894		$	117,461
Furniture, fixtures and equipment	183,801			178,183
Software costs	103,842			93,040
	405,537			388,684
Accumulated depreciation and amortization	(278,455		)	(259,126		)
	127,082			129,558
Land	25,814			24,653
Construction in progress	3,549			3,396
	$	156,445		$	157,607

Depreciation expense was $16.3 million, $17.1 million and $17.8 million, and capitalized software amortization was $9.9 million, $10.6 million and $10.3 million, in 2017, 2016 and 2015, respectively. The unamortized software cost was $22.4 million and $21.6 million as of December 31, 2017 and 2016, respectively. Furniture, fixtures and equipment include capitalized leases of $10.6 million and $12.0 million and related accumulated amortization of $9.0 million and $8.9 million as of December 31, 2017 and 2016, respectively.

DEBT

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

DEBT

7. DEBT

The following table sets forth WESCO’s outstanding indebtedness:



	As of December 31,
	2017			2016
	(In thousands)
International lines of credit	$	34,075		$	20,920
Term Loan Facility, less debt discount of $513 and $770 in 2017 and 2016, respectively	84,237			143,980
Accounts Receivable Securitization Facility	380,000			380,000
Revolving Credit Facility	12,000			4,000
5.375% Senior Notes due 2021	500,000			500,000
5.375% Senior Notes due 2024	350,000			350,000
Capital leases	1,959			2,881
Total debt	1,362,271			1,401,781
Less unamortized debt issuance costs	(13,711		)	(16,508		)
Less short-term debt and current portion of long-term debt	(35,299		)	(22,138		)
Total long-term debt	$	1,313,261		$	1,363,135

International Lines of Credit

Certain foreign subsidiaries of WESCO have entered into uncommitted lines of credit, which serve as overdraft facilities, to support local operations. The maximum borrowing limit varies by facility and ranges between $0.3 million and $21.0 million. The applicable interest rate for borrowings under these lines of credit varies by country and is governed by the applicable loan agreement. The international lines of credit are renewable on an annual basis and certain facilities are fully and unconditionally guaranteed by WESCO Distribution. Accordingly, borrowings under these lines directly reduce availability under the Revolving Credit Facility. The average interest rate for these facilities was 5.42% and 7.36% at December 31, 2017 and 2016, respectively.

Term Loan Facility

On December 12, 2012, WESCO Distribution, as U.S. borrower, WDCC Enterprises Inc. ("WDCC" and together with WESCO Distribution, the “Borrowers”), as Canadian borrower, and WESCO International entered into a Term Loan Agreement (the “Term Loan Agreement”) among WESCO Distribution, WDCC, the Company, the lenders party thereto and Credit Suisse AG Cayman Islands Branch, as administrative agent and as collateral agent.

The Term Loan Agreement provided a seven-year term loan facility (the “Term Loan Facility”), which consisted of two separate sub-facilities: (i) a Canadian sub-facility in an aggregate principal amount of CAD150 million, issued at a 2.0% discount, and (ii) a U.S. sub-facility in an aggregate principal amount of $700 million, issued at a 1.0% discount. The proceeds of the Term Loan Facility were used to finance the acquisition of EECOL, and to pay fees and expenses incurred in connection with the acquisition and certain other transactions. Subject to the terms of the Term Loan Agreement, the Borrowers may request incremental term loans from time to time in an aggregate principal amount not to exceed at any time $300 million, with an equivalent principal amount in U.S. dollars being calculated for any incremental term loan denominated in Canadian dollars.

On November 19, 2013, the Borrowers and WESCO International entered into an amendment (the “Term Loan Amendment”) to the Term Loan Agreement. The Term Loan Amendment, among other things, reduced the applicable margin on U.S. term loans by 0.50% and the LIBOR floor applicable to the U.S. sub-facility from 1.00% to 0.75%. The modified pricing terms were effective December 13, 2013.

On November 26, 2013, WESCO Distribution sold $500 million aggregate principal amount of 5.375% Senior Notes due 2021 (the “2021 Notes”), and used the net proceeds plus excess cash to prepay $500 million under the Company's U.S. sub-facility of the Term Loan Facility (see discussion below under “5.375% Senior Notes due 2021” for additional information). The prepayment satisfied all remaining quarterly repayment obligations under the U.S. sub-facility. As of December 31, 2017, the amount outstanding under the U.S. sub-facility was $84.8 million. The Canadian sub-facility was fully repaid in 2015 using cash provided by Canadian operations.

Borrowings under the Term Loan Facility bear interest at base rates plus applicable margins. At December 31, 2017, the interest rate on borrowings under the U.S. sub-facility was 4.7%. To the extent not previously paid, the outstanding U.S. sub-facility will become due and payable on December 12, 2019, with any unpaid incremental term loans becoming due and payable on the respective maturity dates applicable to those incremental term loans. At any time or from time to time, the Borrowers may prepay borrowings under the Term Loan Facility in whole or in part without premium or penalty. The Borrowers' obligations under the Term Loan Facility are secured by substantially all of the assets of the Borrowers, the Company and certain of the Company's other subsidiaries; provided that, with respect to borrowings under the U.S. sub-facility, the collateral does not include assets of certain foreign subsidiaries or more than 65% of the issued and outstanding equity interests in certain foreign subsidiaries.

The Term Loan Facility contains customary affirmative and negative covenants for credit facilities of this type. The Term Loan Facility also provides for customary events of default.

Accounts Receivable Securitization Facility

On November 8, 2017, WESCO Distribution amended its Receivables Facility pursuant to the terms and conditions of a Fifth Amendment to Fourth Amended and Restated Receivables Purchase Agreement, by and among WESCO Receivables Corp. (“WESCO Receivables”), WESCO Distribution, the various purchasers from time to time party thereto and PNC Bank, National Association, as Administrator (the "Amendment"). The Amendment extended the term of the Receivables Facility to September 24, 2020 and added and amended certain other defined terms. Substantially all other terms and conditions of the Receivables Facility were unchanged.

The Receivables Facility has a purchase limit of $550 million with the opportunity to exercise an accordion feature that permits increases in the purchase limit of up to $100 million. The interest rate spread and commitment fee of the Receivables Facility is 0.95% and 0.45%, respectively.

Under the Receivables Facility, WESCO sells, on a continuous basis, an undivided interest in all domestic accounts receivable to WESCO Receivables, a wholly owned special purpose entity (the “SPE”). The SPE sells, without recourse, a senior undivided interest in the receivables to financial institutions for cash while maintaining a subordinated undivided interest in the receivables, in the form of overcollateralization. WESCO has agreed to continue servicing the sold receivables for the third-party conduits and financial institutions at market rates; accordingly, no servicing asset or liability has been recorded.

As of December 31, 2017 and 2016, accounts receivable eligible for securitization totaled $751.2 million and $657.5 million, respectively. The Consolidated Balance Sheets as of December 31, 2017 and 2016 include $380.0 million of account receivable balances legally sold to third parties in both years, as well as borrowings for equal amounts. At December 31, 2017, the interest rate for this facility was approximately 1.9%.

Revolving Credit Facility

On September 24, 2015, WESCO International, WESCO Distribution and certain other subsidiaries of the Company entered into a $600 million revolving credit facility (the “Revolving Credit Facility”), which contains a letter of credit sub-facility of up to $125 million, pursuant to the terms and conditions of a Second Amended and Restated Credit Agreement (the "Credit Agreement"). The Revolving Credit Facility contains an accordion feature allowing WESCO Distribution to request increases to the borrowing commitments of up to $200 million in the aggregate, subject to customary conditions.

The Revolving Credit Facility matures in September 2020 and consists of two separate sub-facilities: (i) a Canadian sub-facility with a borrowing limit of up to $400 million, which is collateralized by substantially all assets of WESCO Canada and the other Canadian Borrowers, other than, among other things, real property, in each case, subject to customary exceptions and limitations, and (ii) a U.S sub-facility with a borrowing limit of up to $600 million less the amount of outstanding borrowings under the Canadian sub-facility. The U.S. sub-facility is collateralized by substantially all assets of WESCO Distribution and its domestic subsidiaries which are party to the Credit Agreement, other than, among other things, real property and accounts receivable sold or intended to be sold pursuant to the Receivables Purchase Agreement. The applicable interest rate for borrowings under the Revolving Credit Facility includes interest rate spreads based on available borrowing capacity that range between 1.25% and 1.75% for LIBOR-based borrowings and 0.25% and 0.75% for prime rate-based borrowings. At December 31, 2017, the interest rate for this facility was approximately 3.0%.

The Credit Agreement requires ongoing compliance with certain customary affirmative and negative covenants. The Credit Agreement also contains customary events of default.

During 2017, WESCO borrowed $834.4 million under the Revolving Credit Facility and made repayments in the aggregate amount of $826.4 million. During 2016, aggregate borrowings and repayments were $1,025.8 million and $1,096.8 million, respectively. WESCO had $562.9 million available under the Revolving Credit facility at December 31, 2017, after giving effect to $18.0 million of outstanding letters of credit, $19.5 million of surety bonds, and $7.1 million of other reserves, as compared to $509.7 million available under the Revolving Credit facility at December 31, 2016, after giving effect to $20.1 million of outstanding letters of credit, $16.2 million of surety bonds, and $6.4 million of other reserves.

5.375% Senior Notes due 2021

In November 2013, WESCO Distribution issued $500 million aggregate principal amount of 2021 Notes through a private offering exempt from the registration requirements of the Securities Act of 1933, as amended (the “Securities Act”). The 2021 Notes were issued at 100% of par and are governed by an indenture (the “2021 Indenture”) entered into on November 26, 2013 between WESCO International and U.S. Bank National Association, as trustee. The 2021 Notes are unsecured senior obligations of WESCO Distribution and are guaranteed on a senior unsecured basis by WESCO International. The 2021 Notes bear interest at a stated rate of 5.375%, payable semi-annually in arrears on June 15 and December 15 of each year. In addition, WESCO incurred costs related to the issuance of the 2021 Notes totaling $8.4 million, which are recorded as a reduction to the carrying value of the debt and are being amortized over the life of the notes. The 2021 Notes mature on December 15, 2021. The net proceeds of the 2021 Notes were used to prepay a portion of the U.S. sub-facility of the term loans due 2019.

Under the terms of a registration rights agreement dated as of November 26, 2013 among WESCO Distribution, WESCO International and Merrill Lynch, Pierce, Fenner & Smith Incorporated, as the representative of the initial purchasers of the 2021 Notes, WESCO Distribution and WESCO International agreed to register under the Securities Act notes having terms identical in all material respects to the 2021 Notes (the “2021 Exchange Notes”) and to make an offer to exchange the 2021 Exchange Notes for the 2021 Notes. WESCO Distribution launched the exchange offer on June 12, 2014 and the exchange offer expired on July 17, 2014.

At any time WESCO Distribution may redeem all or a part of the 2021 Notes. Between December 15, 2017 and December 14, 2018, WESCO Distribution may redeem all or a part of the 2021 Notes at a redemption price equal to 102.688% of the principal amount. Between December 15, 2018 and December 14, 2019, WESCO Distribution may redeem all or a part of the 2021 Notes at a redemption price equal to 101.344% of the principal amount. On and after December 15, 2019, WESCO Distribution may redeem all or a part of the 2021 Notes at a redemption price equal to 100% of the principal amount.

The 2021 Indenture contains customary covenants and customary events of default. In addition, upon a change of control, the holders of 2021 Notes have the right to require WESCO Distribution to repurchase all or any part of the 2021 Notes at a redemption price equal to 101% of the principal amount, plus accrued and unpaid interest.

5.375% Senior Notes due 2024

In June 2016, WESCO Distribution issued $350 million aggregate principal amount of 5.375% Senior Notes due 2024 (the "2024 Notes") through a private offering exempt from the registration requirements of the Securities Act. The 2024 Notes were issued at 100% of par and are governed by an indenture (the “2024 Indenture”) entered into on June 15, 2016 among WESCO Distribution, as issuer, WESCO International, as parent guarantor, and U.S. Bank National Association, as trustee. The 2024 Notes are unsecured senior obligations of WESCO Distribution and are guaranteed on a senior unsecured basis by WESCO International. The 2024 Notes bear interest at a rate of 5.375% per annum, payable semi-annually in arrears on June 15 and December 15 of each year, commencing on December 15, 2016. WESCO incurred costs totaling $6.0 million to issue the 2024 Notes, which are recorded as a reduction to the carrying value of the debt and are being amortized over the life of the note. The notes mature on June 15, 2024. The Company used the net proceeds to redeem its 2029 Debentures on September 15, 2016.

Under the terms of a registration rights agreement dated as of June 15, 2016 among WESCO Distribution, as the issuer, WESCO International, as parent guarantor, and Goldman, Sachs & Co., as representative of the initial purchasers of the 2024 Notes, WESCO Distribution and WESCO International agreed to register under the Securities Act notes having terms identical in all material respects to the 2024 Notes (the “2024 Exchange Notes”) and to make an offer to exchange the 2024 Exchange Notes for the 2024 Notes. WESCO Distribution launched the exchange offer on December 28, 2016 and the exchange offer expired on January 31, 2017.

At any time on or after June 15, 2019, WESCO Distribution may redeem all or a part of the 2024 Notes. Between June 15, 2019 and June 14, 2020, WESCO Distribution may redeem all or a part of the 2024 Notes at a redemption price equal to 104.031% of the principal amount. Between June 15, 2020 and June 14, 2021, WESCO Distribution may redeem all or a part of the 2024 Notes at a redemption price equal to 102.688% of the principal amount. Between June 15, 2021 and June 14, 2022, WESCO Distribution may redeem all or a part of the 2024 Notes at a redemption price equal to 101.344% of the principal amount. On and after June 15, 2022, WESCO Distribution may redeem all or a part of the 2024 Notes at a redemption price equal to 100% of the principal amount.

The 2024 Indenture contains customary covenants and events of default. Upon a change of control, the holders of the 2024 Notes have the right to require WESCO Distribution to repurchase all or any part of the 2024 Notes at a redemption price equal to 101% of the principal amount thereof, plus accrued and unpaid interest.

6.0% Convertible Senior Debentures due 2029

On August 27, 2009, WESCO International completed an exchange offer pursuant to which it issued $345 million in aggregate principal amount of the 2029 Debentures. On September 15, 2016, the Company redeemed the 2029 Debentures. Holders of the 2029 Debentures received cash totaling $344.8 million, which was equal to the principal amount of the then-outstanding debentures, in addition to accrued and unpaid interest. Holders who surrendered the 2029 Debentures for conversion received 18 shares of WESCO stock for each $1,000 principal amount of 2029 Debentures converted. In total, 6,267,688 shares were issued on the redemption date. The redemption resulted in a non-cash loss of $123.9 million, which included the write off of unamortized debt issuance costs.

WESCO separately accounted for the liability and equity components of its 2029 Debentures in a manner that reflected its non-convertible debt borrowing rate. WESCO utilized an interest rate of 13.875% to reflect the non-convertible debt borrowing rate of its offering upon issuance, which was determined based on discussions with its financial institutions and a review of relevant market data, and resulted in a $181.2 million discount to the 2029 Debenture balance and a net increase in additional capital of $106.5 million. In addition, the financing costs incurred to issue the 2029 Debentures were allocated between the instrument's debt and equity components. WESCO amortized the debt discount and financing costs over the life of the instrument. For the years ended December 31, 2016 and 2015, non-cash interest expense for the amortization of the debt discount and debt issuance costs was $3.1 million and $4.2 million, respectively.

Covenant Compliance

WESCO was in compliance with all relevant covenants contained in its debt agreements as of December 31, 2017.

The following table sets forth the aggregate principal repayment requirements for all indebtedness for the next five years and thereafter, as of December 31, 2017:



	(In thousands)
2018	$	35,299
2019	85,246
2020	392,239
2021	500,000
2022	—
Thereafter	350,000
Total payments on debt	1,362,784
Debt discount	(513		)
Total debt	$	1,362,271

WESCO’s credit agreements contain various restrictive covenants that, among other things, impose limitations on: (i) dividend payments or certain other restricted payments or investments; (ii) the incurrence of additional indebtedness and guarantees; (iii) creation of liens; (iv) mergers, consolidation or sales of substantially all of WESCO’s assets; (v) certain transactions among affiliates; (vi) payments by certain subsidiaries to WESCO, and (vii) capital expenditures. In addition, the Revolving Credit Facility and the Receivables Facility require WESCO to meet certain fixed charge coverage tests depending on availability or liquidity, respectively.

CAPITAL STOCK	12 Months Ended
CAPITAL STOCK	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
CAPITAL STOCK	8. CAPITAL STOCK Preferred Stock There are 20 million shares of preferred stock authorized at a par value of $0.01 per share; there are no shares issued or outstanding. The Board of Directors has the authority, without further action by the stockholders, to issue all authorized preferred shares in one or more series and to fix the number of shares, designations, voting powers, preferences, optional and other special rights and the restrictions or qualifications thereof. The rights, preferences, privileges and powers of each series of preferred stock may differ with respect to dividend rates, liquidation values, voting rights, conversion rights, redemption provisions and other matters. Common Stock There are 210 million shares of common stock and 20 million shares of Class B common stock authorized at a par value of $0.01 per share. The Class B common stock is identical to the common stock, except for voting and conversion rights. The holders of Class B common stock have no voting rights. With certain exceptions, Class B common stock may be converted, at the option of the holder, into the same number of shares of common stock. The terms of the Revolving Credit Facility and the Term Loan Facility provide certain limits on declaring or paying dividends, and repurchasing common stock. In addition, the indentures governing the 2021 Notes and 2024 Notes place limits on the Company's ability to pay dividends and repurchase common stock. At December 31, 2017 and 2016, no dividends had been declared and, therefore, no retained earnings were reserved for dividend payments.

INCOME TAXES

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

INCOME TAXES

9. INCOME TAXES

The Tax Cuts and Jobs Act of 2017 (the "TCJA”), enacted on December 22, 2017, provides a broad range of tax reform, including changes to the U.S. corporate tax rate, business-related exclusions, deductions and credits, as well as international tax provisions. Most notably, the TCJA permanently reduces the U.S. corporate income tax rate from 35% to 21%, effective January 1, 2018, and imposes a one-time tax on the deemed repatriation of undistributed foreign earnings (the "transition tax"). The TCJA also introduces anti-base erosion provisions, including the global intangible low-taxed income ("GILTI") tax.

As a result of the reduction in the U.S. corporate income tax rate, the Company remeasured its U.S. deferred income tax balances as of December 31, 2017 and recorded a provisional deferred income tax benefit of $56.4 million for the year ended December 31, 2017. The Company also recognized provisional current income tax expense for the transition tax under the TCJA of $82.8 million for the year ended December 31, 2017. After the utilization of foreign tax credit carryforwards of $17.8 million, a provisional liability of $65.0 million was accrued for the transition tax as of December 31, 2017 and is payable over a period of eight years.

As of December 31, 2017, the Company had not completed its accounting for certain income tax effects of the TCJA and the provisional amounts described above could change materially in future periods. Although the Company made reasonable estimates to determine the effect of the TCJA on its deferred income taxes, the Company must gather and analyze additional information to complete the related accounting. Similarly, the Company's accounting for the income tax effects of the transition tax is incomplete primarily due to the complexity of computing foreign earnings and measuring the portion of such earnings held in cash and cash equivalents, which is subject to a higher tax rate. Further regulatory guidance is expected to be issued regarding the implementation and interpretation of the TCJA provisions, which could affect the Company's analyses and ultimate conclusions. As described in Note 2, future adjustments (if any) will be recognized as discrete income tax expense or benefit in the period the adjustments are determined. The accounting for the income tax effects of the TCJA will be completed within the measurement period defined in Note 2.

The following table sets forth the components of income before income taxes by jurisdiction:



	Year Ended December 31,
	2017		2016		2015
			(In thousands)
United States	$	180,957	$	80,881	$	288,881
Foreign	71,483		50,670		15,029
Income before income taxes	$	252,440	$	131,551	$	303,910

The following table sets forth the components of the provision (benefit) for income taxes:



	Year Ended December 31,
	2017			2016			2015
				(In thousands)
Current income taxes:
Federal ⁽¹⁾	$	122,170		$	65,614		$	45,812
State	2,259			6,489			4,565
Foreign	15,274			3,502			2,309
Total current income taxes	139,703			75,605			52,686
Deferred income taxes:
Federal	(48,060		)	(42,835		)	29,593
State	4,508			(2,938		)	3,767
Foreign	(6,844		)	599			9,491
Total deferred income taxes	(50,396		)	(45,174		)	42,851
Provision for income taxes	$	89,307		$	30,431		$	95,537


⁽¹⁾	Income tax (expense) benefit related to stock-based awards and other equity instruments recorded directly to additional paid in capital totaled $(0.1) million and $1.6 million in 2016 and 2015, respectively. Due to the adoption of ASU 2016-09, as described in Note 2, there was no income tax (expense) benefit recorded to additional paid in capital for stock-based awards in 2017.

The following table sets forth the reconciliation between the federal statutory income tax rate and the effective tax rate:



	Year Ended December 31,
	2017		2016		2015
Federal statutory rate	35.0	%	35.0	%	35.0	%
State income taxes, net of federal income tax benefit	1.4		1.0		2.2
Deemed repatriation of undistributed foreign earnings	32.8		—		—
Deferred income tax remeasurement	(22.4	)	—		—
Tax effect of intercompany financing	(10.5	)	(19.9	)	(8.8	)
Foreign tax rate differences	(1.3	)	(0.4	)	(1.1	)
Valuation allowance against deferred tax assets	0.4		1.1		—
Nondeductible expenses	(0.1	)	1.6		1.2
Adjustment related to uncertain tax positions	—		3.7		2.7
Other	0.1		1.0		0.2
Effective tax rate	35.4	%	23.1	%	31.4	%

As of December 31, 2017, WESCO’s foreign subsidiaries had undistributed earnings of approximately $884.0 million, of which $807.2 million was attributable to the Company's Canadian operations. As described above, WESCO recognized provisional income tax expense of $82.8 million for the year ended December 31, 2017 resulting from the transition tax. Notwithstanding the effects of applying such provisions of the TCJA, WESCO continues to assert that the earnings of its foreign subsidiaries are indefinitely reinvested to fund growth in the foreign markets. However, as a result of the TCJA, the Company is reevaluating its intent and ability to repatriate foreign cash based upon the available liquidity and cash flow needs of its foreign subsidiaries and will disclose in future filings any change in its intention to repatriate undistributed foreign earnings and any resulting income tax impacts. Until the Company completes this reevaluation, it is not practicable to determine the amount of any unrecognized deferred income taxes on these undistributed foreign earnings.

The following table sets forth deferred tax assets and liabilities:



	As of December 31,
	2017					2016
				(In thousands)
	Assets			Liabilities		Assets			Liabilities
Accounts receivable	$	3,496		$	—	$	3,484		$	—
Inventories	—			3,181		—			4,001
Depreciation of property, buildings and equipment	—			13,283		—			11,487
Amortization of intangible assets ⁽¹⁾	—			159,107		—			237,015
Employee benefits	14,835			—		18,577			—
Stock-based compensation ⁽²⁾	16,341			—		23,844			—
Advance payments	8,456			—		22,056			—
Foreign tax credits	—			—		15,698			—
Tax loss carryforwards	19,128			—		18,440			—
Other	11,850			8,672		7,175			7,783
Deferred income taxes before valuation allowance	74,106			184,243		109,274			260,286
Valuation allowance	(2,518		)	—		(1,430		)	—
Total deferred income taxes ⁽¹⁾	$	71,588		$	184,243	$	107,844		$	260,286


⁽¹⁾	As described in Note 2, the Consolidated Balance Sheet at December 31, 2016 was revised to correct certain financial statement line items, including deferred income taxes.


⁽²⁾	The Company does not expect the realizability of the deferred tax asset related to stock-based compensation to be materially impacted by the TCJA's expansion on the limitation of deductions for excessive employee compensation.

As of December 31, 2017 and 2016, WESCO had deferred tax assets of $10.4 million and $10.0 million, respectively, related to Canadian net operating loss carryforwards. The Canadian net operating loss carryforwards expire beginning in 2029 through 2037. Additionally, WESCO had deferred tax assets of $7.0 million and $6.2 million as of December 31, 2017 and 2016, respectively, related to non-Canadian foreign net operating loss carryforwards. These net operating loss carryforwards expire beginning in 2019, while some may be carried forward indefinitely. As of December 31, 2017 and 2016, WESCO had deferred tax assets of $3.1 million and $3.2 million, respectively, related to state net operating loss carryforwards. These carryforwards expire beginning in 2022 through 2036. The Company has determined, based upon an evaluation of all available evidence, that it "more-likely-than-not" will utilize all of its net operating loss carryforwards before expiration other than those incurred in a non-Canadian foreign location. Accordingly, the Company recorded a full valuation allowance against the total deferred tax asset related to these non-Canadian foreign net operating loss carryforwards of $2.5 million at December 31, 2017.

As of December 31, 2016, WESCO had deferred tax assets of $15.7 million related to foreign tax credit (“FTC”) carryforwards. As described above, the Company fully utilized these FTC carryforwards against the liability recorded for the transition tax imposed by the TCJA.

The Company is under examination by tax authorities in the U.S. and Canada and remains subject to examination until the applicable statutes of limitation expire. The statutes of limitation for the material jurisdictions in which the Company files income tax returns remain open as follows:



United States — Federal		2004 and forward
United States — Material States		2013 and forward
Canada		2004 and forward

The statutes of limitation with respect to the Company’s 2004 to 2007 U.S. federal income tax returns are open by waiver only in connection with the implementation of the Mutual Agreement Procedure (“MAP”) concluded in the fourth quarter of 2015 between the Competent Authorities of the Internal Revenue Service (the "IRS") and the Canada Revenue Agency (the “CRA”) and the request for MAP assistance filed with the IRS and CRA in 2017 with respect to certain other 2004 to 2007 transfer pricing matters. The statutes of limitation with respect to the Company’s 2008 to 2011 U.S. federal income tax returns are open by waiver only in connection with the implementation of the Advance Pricing Agreement (“APA”) concluded in the fourth quarter of 2015 between the IRS and CRA and certain other transfer pricing matters pending with the CRA. The statutes of limitation with respect to the Company’s 2012 and 2013 U.S. federal income tax returns are open by waiver only in connection with the IRS examination of the 2012 and 2013 years and the APA.

The following table sets forth the reconciliation of gross unrecognized tax benefits:



	As of December 31,
	2017			2016			2015
	(In thousands)
Beginning balance January 1	$	6,181		$	5,436		$	20,033
Additions based on tax positions related to the current year	—			—			46
Additions for tax positions of prior years	—			3,298			402
Reductions for tax positions of prior years	(155		)	(21		)	(378		)
Settlements	(1,025		)	(1,921		)	(9,638		)
Lapse in statute of limitations	(755		)	(728		)	(1,497		)
Foreign currency exchange rate changes	102			117			(3,532		)
Ending balance December 31	$	4,348		$	6,181		$	5,436

The total amount of unrecognized tax benefits were $4.3 million, $6.2 million, and $5.4 million as of December 31, 2017, 2016 and 2015, respectively. The amount of unrecognized tax benefits that would affect the effective tax rate if recognized in the consolidated financial statements was $1.7 million, $7.5 million, and $6.2 million, respectively.

It is reasonably possible that the amount of unrecognized tax benefits will decrease by approximately $3.0 million within the next twelve months due to the settlement of uncertain tax positions related to IRS audits or the expiration of statutes of limitation. Of this amount, approximately $0.3 million could impact the effective tax rate.

The Company classifies interest related to unrecognized tax benefits as interest income or expense. Interest expense on unrecognized tax benefits was $0.1 million and $1.2 million for 2017 and 2016, respectively. In 2015, interest income of $8.7 million was recognized as a result of the conclusion of the MAP and APA proceedings for the 2004 to 2014 tax years. As of December 31, 2017 and 2016, WESCO had an accrued liability of $1.8 million and $2.2 million, respectively, for interest expense related to unrecognized tax benefits. The Company classifies penalties related to unrecognized tax benefits as part of income tax expense. Penalties recorded in income tax expense were immaterial in 2017, 2016, and 2015.

EARNINGS PER SHARE EARNINGS PER SHARE

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

EARNINGS PER SHARE

10. EARNINGS PER SHARE

Basic earnings per share is computed by dividing net income attributable to WESCO International by the weighted-average number of common shares outstanding during the periods. Diluted earnings per share is computed by dividing net income attributable to WESCO International by the weighted-average common shares and common share equivalents outstanding during the periods. The dilutive effect of common share equivalents is considered in the diluted earnings per share computation using the treasury stock method, which includes consideration of equity awards and contingently convertible debt.

The following tables set forth the details of basic and diluted earnings per share:



	Year Ended December 31,
	2017		2016		2015
(In thousands, except per share data)
Net income attributable to WESCO International	$	163,460	$	101,588	$	210,687
Weighted-average common shares outstanding used in computing basic earnings per share	47,849		44,116		43,433
Common shares issuable upon exercise of dilutive equity awards	512		543		626
Common shares issuable from contingently convertible debentures (see below for basis of calculation)	—		3,674		6,314
Weighted-average common shares outstanding and common share equivalents used in computing diluted earnings per share	48,361		48,333		50,373
Earnings per share attributable to WESCO International
Basic	$	3.42	$	2.30	$	4.85
Diluted	$	3.38	$	2.10	$	4.18

The computation of diluted earnings per share attributable to WESCO International excluded equity awards of approximately 1.3 million for the year ended December 31, 2017 and approximately 1.2 million for the years ended December 31, 2016 and 2015. These shares were excluded because their effect would have been antidilutive.

Because of WESCO’s previous obligation to settle the par value of the 2029 Debentures in cash upon conversion, WESCO was required to include shares underlying the 2029 Debentures in its diluted weighted-average shares outstanding when the average stock price per share for the period exceeded the conversion price of the debentures. Only the number of shares that would have been issuable under the treasury stock method of accounting for share dilution were included, which was based upon the amount by which the average stock price exceeded the conversion price. The conversion price of the 2029 Debentures was $28.87 and the maximum amount of share dilution was limited to 11,951,932 shares. Since the 2029 Debentures were redeemed on September 15, 2016, there was no dilution from contingently convertible debentures for the year ended December 31, 2017. For the years ended December 31, 2016 and 2015, the effect of the 2029 Debentures on diluted earnings per share attributable to WESCO International was a decrease of $0.17 and $0.60, respectively.

In December 2014, the Company's Board of Directors authorized the repurchase of up to $300 million of the Company's common stock through December 31, 2017. As of December 31, 2016, WESCO had repurchased 2,468,576 shares of the Company's common stock for $150.0 million under this repurchase authorization. During the year ended December 31, 2017, the Company entered into accelerated stock repurchase agreements (the "ASR Transactions") with a certain financial institution to repurchase additional shares of its common stock. In exchange for up-front cash payments totaling $100.0 million, the Company received 1,778,537 shares. The total number of shares ultimately delivered under the ASR Transactions was determined by the average of the volume-weighted-average prices of the Company's common stock for each exchange business day during the respective settlement valuation periods. WESCO funded the repurchases with borrowings under its accounts receivable securitization and revolving credit facilities. For purposes of computing earnings per share, share repurchases have been reflected as a reduction to common shares outstanding on the respective delivery dates.

STOCK-BASED COMPENSATION STOCK-BASED COMPENSATION

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

STOCK-BASED COMPENSATION

12. STOCK-BASED COMPENSATION

WESCO sponsors four stock-based compensation plans. The 1999 Long-Term Incentive Plan, as amended and restated (“LTIP”), was designed to be the successor plan to all prior plans. Any shares remaining reserved for future issuance under the prior plans are available for issuance under the LTIP. The LTIP and predecessor plans are administered by the Compensation Committee of the Board of Directors.

On May 31, 2017, the Company renewed and restated the LTIP, increasing the maximum number of shares of common stock that may be issued under the plan by 1.7 million shares to 3.4 million. Under the LTIP, the total number of shares of common stock authorized to be issued will be reduced by 1 share of common stock for every 1 share that is subject to a stock appreciation right granted, and 1.83 shares of common stock for every 1 share that is subject to an award other than a stock appreciation right granted on or after May 31, 2017. As of December 31, 2017, 3.6 million shares of common stock were reserved under the LTIP for future equity award grants.

Except for the performance-based award, awards granted vest and become exercisable once criteria based on time is achieved. Performance-based awards vest based on market or performance conditions. All awards vest immediately in the event of a change in control. Each award terminates on the tenth anniversary of its grant date unless terminated sooner under certain conditions.

WESCO recognized $14.8 million, $12.5 million and $12.9 million of non-cash stock-based compensation expense, which is included in selling, general and administrative expenses, for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017, there was $18.7 million of total unrecognized compensation expense related to non-vested stock-based compensation arrangements for all awards previously made of which approximately $11.3 million is expected to be recognized in 2018, $6.6 million in 2019 and $0.8 million in 2020.

The total intrinsic value of awards exercised during the years ended December 31, 2017, 2016, and 2015 was $17.2 million, $13.0 million, and $15.8 million, respectively. The gross deferred tax benefit associated with the exercise of stock-based awards totaled $6.4 million, $4.9 million, and $5.7 million in 2017, 2016, and 2015, respectively.

The following table sets forth a summary of stock-settled stock appreciation rights and related information for the years indicated:



	2017							2016				2015
	Awards		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Life	Aggregate Intrinsic Value (In thousands)		Awards		Weighted-Average Exercise Price		Awards		Weighted-Average Exercise Price
Beginning of year	2,439,487		$	52.62				2,567,021		$	54.47	2,480,745		$	50.91
Granted	455,807		71.21					709,999		42.63		394,182		69.54
Exercised	(495,181	)	42.19					(526,818	)	41.54		(232,542	)	35.80
Canceled	(161,506	)	66.06					(310,715	)	63.71		(75,364	)	73.59
End of year	2,238,607		57.75		6.2	$	28,791	2,439,487		52.62		2,567,021		54.47
Exercisable at end of year	1,331,580		$	56.96	4.6	$	18,801	1,549,350		$	53.35	2,034,263		$	49.36

WESCO granted the following stock-settled stock appreciation rights at the following weighted-average assumptions:



	2017	2016	2015
Stock-settled stock appreciation rights granted	455,807	709,999	394,182
Risk free interest rate	1.9%	1.2%	1.6%
Expected life (in years)	5	5	5
Expected volatility	29%	32%	32%

The weighted-average fair value per stock-settled stock appreciation right granted was $20.52, $12.88 and $21.68 for the years ended December 31, 2017, 2016 and 2015, respectively.

The following table sets forth a summary of time-based restricted stock units and related information for the years ended December 31, 2017, 2016 and 2015:



	2017				2016				2015
	Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value
Unvested at beginning of year	257,096		$	57.47	175,411		$	74.52	185,457		$	73.87
Granted	100,993		71.33		162,256		44.45		81,022		69.05
Vested	(44,720	)	84.57		(60,015	)	72.41		(76,387	)	66.89
Forfeited	(23,315	)	57.52		(20,556	)	59.15		(14,681	)	75.73
Unvested at end of year	290,054		$	58.11	257,096		$	57.47	175,411		$	74.52

The weighted-average fair value per restricted stock unit granted was $71.33, $44.45 and $69.05 for the years ended December 31, 2017, 2016 and 2015, respectively.

The following table sets forth a summary of performance-based awards for the year ended December 31, 2017:



	2017				2016				2015
	Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value
Unvested at beginning of year	149,320		$	60.36	114,520		$	76.48	130,004		$	80.21
Granted	39,978		76.63		91,768		47.00		59,661		67.81
Vested	—		—		—		—		(38,869	)	72.25
Forfeited	(40,790	)	76.77		(56,968	)	71.25		(36,276	)	80.14
Unvested at end of year	148,508		$	60.23	149,320		$	60.36	114,520		$	76.48

The weighted-average fair value per performance-based award granted was $76.63, $47.00 and $67.81 for the years ended December 31, 2017, 2016 and 2015, respectively.

The fair value of the performance shares based on total stockholder return granted during the year ended December 31, 2017, 2016 and 2015 were estimated using the following weighted-average assumptions:



	Year ended December 31,
	2017			2016			2015
Grant date share price	$	71.65		$	42.44		$	69.54
WESCO expected volatility	29		%	26		%	27		%
Peer group median volatility	24		%	24		%	23		%
Risk-free interest rate	1.5		%	0.9		%	1.1		%
Correlation	114		%	122		%	96		%

The unvested performance-based awards in the table above include 74,254 shares in which vesting of the ultimate number of shares is dependent upon WESCO's total stockholder return in relation to the total stockholder return of a select group of peer companies over a three-year period. The fair value of these awards is determined using a Monte Carlo simulation model. These awards are accounted for as awards with market conditions; compensation cost is recognized over the service period, regardless of whether the market conditions are achieved and the awards ultimately vest.

Vesting of the remaining 74,254 shares of performance-based awards in the table above is dependent upon the three-year average growth rate of WESCO's net income. The fair value of these awards is based upon the grant-date closing price of WESCO's common stock. These awards are accounted for as awards with performance conditions; compensation cost is recognized over the performance period based upon WESCO's determination of whether it is probable that the performance targets will be achieved.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

13. COMMITMENTS AND CONTINGENCIES

Future minimum rental payments required under operating leases, primarily for real property that have noncancelable lease terms in excess of one year as of December 31, 2017, are as follows:



Years ending December 31	(In thousands)
2018	$	65,510
2019	55,378
2020	44,036
2021	33,498
2022	23,904
Thereafter	49,150

Rental expense for the years ended December 31, 2017, 2016 and 2015 was $82.0 million, $76.7 million and $70.7 million, respectively.

From time to time, a number of lawsuits and claims have been or may be asserted against us relating to the conduct of its business, including routine litigation relating to commercial and employment matters. The outcome of any litigation cannot be predicted with certainty, and some lawsuits may be determined adversely to WESCO. However, management does not believe that the ultimate outcome of any such pending matters is likely to have a material adverse effect on WESCO's financial condition or liquidity, although the resolution in any fiscal period of one or more of these matters may have a material adverse effect on WESCO's results of operations for that period.

WESCO is subject to the laws and regulations of states and other jurisdictions concerning the identification, reporting and escheatment (the transfer of property to the state) of unclaimed or abandoned funds, and is subject to audit and examination for compliance with these requirements. The State of Delaware conducted an audit concerning the identification, reporting and escheatment of unclaimed or abandoned property, and in December 2017, WESCO reached agreement in principle with the State of Delaware to resolve the audit. The settlement amount was not material to the Company’s financial condition or results of operations.

In October 2014, WESCO was notified that the New York County District Attorney’s Office was conducting an investigation involving minority and disadvantaged business contracting practices in the construction industry in New York City and that various contractors, minority and disadvantaged business firms, and their material suppliers, including the Company, were part of this investigation. The Company cooperated with the government investigation and, in November 2017, reached an agreement in principle for a civil settlement to resolve the Company’s involvement in the investigation. The settlement amount was not material to the Company’s financial condition or results of operations.

SEGMENTS AND RELATED INFORMATION

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

SEGMENTS AND RELATED INFORMATION

14. SEGMENTS AND RELATED INFORMATION

WESCO provides distribution of product and services through its four operating segments, which have been aggregated as one reportable segment. WESCO has approximately 230,000 unique product stock keeping units and markets more than 1,000,000 products for customers. There were no material amounts of sales or transfers among geographic areas and no material amounts of export sales.

WESCO attributes revenues from external customers to individual countries on the basis of the point of sale. The following table sets forth information about WESCO by geographic area:



	Net Sales Year Ended December 31,												Long-Lived Assets December 31,
	2017				2016				2015				2017		2016		2015
(In thousands)
United States	$	5,775,988	75	%	$	5,635,803	77	%	$	5,665,962	75	%	$	95,851	$	123,465	$	157,570
Canada	1,521,378		20	%	1,394,657		19	%	1,533,705		21	%	56,591		60,372		63,088
Mexico	77,280		1	%	62,430		1	%	70,048		1	%	262		227		332
Subtotal North American Operations	7,374,646				7,092,890				7,269,715				152,704		184,064		220,990
Other International	304,375		4	%	243,127		3	%	248,772		3	%	3,741		4,583		5,369
Total	$	7,679,021			$	7,336,017			$	7,518,487			$	156,445	$	188,647	$	226,359

The following table sets forth sales information about WESCO’s sales by product category:



Year Ended December 31,	2017	2016	2015
(percentages based on total sales)
General Supplies	40%	40%	40%
Wire, Cable and Conduit	15%	14%	15%
Communications and Security	15%	15%	15%
Electrical Distribution and Controls	10%	11%	11%
Lighting and Sustainability	12%	12%	10%
Automation, Controls and Motors	8%	8%	9%

OTHER FINANCIAL INFORMATION

12 Months Ended

Dec. 31, 2017

Condensed Financial Information [Abstract]

OTHER FINANCIAL INFORMATION

15. CONDENSED CONSOLIDATING FINANCIAL INFORMATION

WESCO Distribution has outstanding $500 million in aggregate principal amount of 2021 Notes and $350 million in aggregate principal amount of 2024 Notes. The 2021 Notes and 2024 Notes are unsecured senior obligations of WESCO Distribution and are fully and unconditionally guaranteed on a senior unsecured basis by WESCO International.

Condensed consolidating financial information for WESCO International, WESCO Distribution and the non-guarantor subsidiaries is presented in the following tables.



	Condensed Consolidating Balance Sheet
	December 31, 2017
	(In thousands)
	WESCO International, Inc.		WESCO Distribution, Inc.		Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Cash and cash equivalents	$	—	$	50,602	$	67,351		$	—		$	117,953
Trade accounts receivable, net	—		—		1,170,080			—			1,170,080
Inventories	—		430,092		526,056			—			956,148
Prepaid expenses and other current assets	4,730		42,547		152,531			(35,140		)	164,668
Total current assets	4,730		523,241		1,916,018			(35,140		)	2,408,849
Intercompany receivables, net	—		—		2,189,136			(2,189,136		)	—
Property, buildings and equipment, net	—		50,198		106,247			—			156,445
Intangible assets, net	—		2,770		364,334			—			367,104
Goodwill	—		257,623		1,514,254			—			1,771,877
Investments in affiliates	3,058,613		5,023,826		—			(8,082,439		)	—
Other assets	—		2,778		28,415			—			31,193
Total assets	$	3,063,343	$	5,860,436	$	6,118,404		$	(10,306,715	)	$	4,735,468

Accounts payable	$	—	$	417,690	$	381,830		$	—		$	799,520
Short-term debt	—		—		34,075			—			34,075
Other current liabilities	—		80,039		162,475			(35,140		)	207,374
Total current liabilities	—		497,729		578,380			(35,140		)	1,040,969
Intercompany payables, net	939,784		1,249,352		—			(2,189,136		)	—
Long-term debt	—		934,033		379,228			—			1,313,261
Other noncurrent liabilities	3,820		120,709		140,566			—			265,095
Total WESCO International stockholders’ equity	2,119,739		3,058,613		5,023,826			(8,082,439		)	2,119,739
Noncontrolling interests	—		—		(3,596		)	—			(3,596		)
Total liabilities and stockholders’ equity	$	3,063,343	$	5,860,436	$	6,118,404		$	(10,306,715	)	$	4,735,468



	Condensed Consolidating Balance Sheet
	December 31, 2016
	(In thousands)
	WESCO International, Inc.		WESCO Distribution, Inc.		Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Cash and cash equivalents	$	—	$	41,552	$	68,579		$	—		$	110,131
Trade accounts receivable, net	—		—		1,034,402			—			1,034,402
Inventories	—		364,562		456,879			—			821,441
Prepaid expenses and other current assets	13,647		34,833		211,637			(123,013		)	137,104
Total current assets	13,647		440,947		1,771,497			(123,013		)	2,103,078
Intercompany receivables, net	—		—		2,060,336			(2,060,336		)	—
Property, buildings and equipment, net	—		51,824		105,783			—			157,607
Intangible assets, net	—		3,417		389,945			—			393,362
Goodwill	—		257,623		1,473,327			—			1,730,950
Investments in affiliates	3,538,476		4,028,502		—			(7,566,978		)	—
Other assets	—		23,846		22,998			—			46,844
Total assets	$	3,552,123	$	4,806,159	$	5,823,886		$	(9,750,327	)	$	4,431,841

Accounts payable	$	—	$	381,795	$	302,926		$	—		$	684,721
Short-term debt	—		—		20,920			—			20,920
Other current liabilities	—		120,299		170,872			(123,013		)	168,158
Total current liabilities	—		502,094		494,718			(123,013		)	873,799
Intercompany payables, net	1,572,486		487,850		—			(2,060,336		)	—
Long-term debt	—		983,449		379,686			—			1,363,135
Other noncurrent liabilities	12,737		55,898		162,641			—			231,276
Total WESCO International stockholders’ equity	1,966,900		2,776,868		4,790,110			(7,566,978		)	1,966,900
Noncontrolling interests	—		—		(3,269		)	—			(3,269		)
Total liabilities and stockholders’ equity	$	3,552,123	$	4,806,159	$	5,823,886		$	(9,750,327	)	$	4,431,841



	Condensed Consolidating Statement of Income and Comprehensive Income
	Year ended December 31, 2017
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net sales	$	—		$	3,370,088		$	4,441,655		$	(132,722	)	$	7,679,021
Cost of goods sold (excluding depreciation and	—			2,714,511			3,612,577			(132,722		)	6,194,366
amortization)
Selling, general and administrative expenses	—			555,503			544,245			—			1,099,748
Depreciation and amortization	—			18,442			45,575			—			64,017
Results of affiliates’ operations	160,587			168,782			—			(329,369		)	—
Interest expense (income), net	—			94,313			(25,863		)	—			68,450
Provision for income taxes	(2,546		)	(4,486		)	96,339			—			89,307
Net income	163,133			160,587			168,782			(329,369		)	163,133
Less: Net loss attributable to noncontrolling interests	—			—			(327		)	—			(327		)
Net income attributable to WESCO International	$	163,133		$	160,587		$	169,109		$	(329,369	)	$	163,460
Other comprehensive income (loss):
Foreign currency translation adjustments	85,762			85,762			85,762			(171,524		)	85,762
Post retirement benefit plan adjustments	(6,381		)	(6,381		)	(6,381		)	12,762			(6,381		)
Comprehensive income attributable to WESCO International	$	242,514		$	239,968		$	248,490		$	(488,131	)	$	242,841



	Condensed Consolidating Statement of Income and Comprehensive Income
	Year ended December 31, 2016
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net sales	$	—		$	3,306,265		$	4,134,508		$	(104,756	)	$	7,336,017
Cost of goods sold (excluding depreciation and	—			2,651,409			3,341,161			(104,756		)	5,887,814
amortization)
Selling, general and administrative expenses	61			477,437			571,788			—			1,049,286
Depreciation and amortization	—			20,226			46,632			—			66,858
Results of affiliates’ operations	240,571			155,814			—			(396,385		)	—
Interest expense (income), net	17,555			87,824			(28,804		)	—			76,575
Loss on debt redemption	123,933			—			—			—			123,933
Provision for income taxes	(2,098		)	8,263			24,266			—			30,431
Net income	101,120			216,920			179,465			(396,385		)	101,120
Less: Net loss attributable to noncontrolling interests	—			—			(468		)	—			(468		)
Net income attributable to WESCO International	$	101,120		$	216,920		$	179,933		$	(396,385	)	$	101,588
Other comprehensive income (loss):
Foreign currency translation adjustments	38,275			38,275			38,275			(76,550		)	38,275
Post retirement benefit plan adjustments	(2,485		)	(2,485		)	(2,485		)	4,970			(2,485		)
Comprehensive income attributable to WESCO International	$	136,910		$	252,710		$	215,723		$	(467,965	)	$	137,378



	Condensed Consolidating Statement of Income and Comprehensive Income (Loss)
	Year ended December 31, 2015
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net sales	$	—		$	3,456,883		$	4,177,383		$	(115,779	)	$	7,518,487
Cost of goods sold (excluding depreciation and	—			2,784,413			3,356,192			(115,779		)	6,024,826
amortization)
Selling, general and administrative expenses	26			611,549			443,376			—			1,054,951
Depreciation and amortization	—			19,703			45,265			—			64,968
Results of affiliates’ operations	225,370			219,619			—			(444,989		)	—
Interest expense (income), net	24,910			63,261			(18,339		)	—			69,832
Provision for income taxes	(7,939		)	(6,929		)	110,405			—			95,537
Net income	$	208,373		$	204,505		$	240,484		$	(444,989	)	$	208,373
Less: Net loss attributable to noncontrolling interests	—			—			(2,314		)	—			(2,314		)
Net income attributable to WESCO International	$	208,373		$	204,505		$	242,798		$	(444,989	)	$	210,687
Other comprehensive income (loss):
Foreign currency translation adjustments	(225,795		)	(225,795		)	(225,795		)	451,590			(225,795		)
Post retirement benefit plan adjustments	4,532			4,532			4,532			(9,064		)	4,532
Comprehensive (loss) income attributable to WESCO International	$	(12,890	)	$	(16,758	)	$	21,535		$	(2,463	)	$	(10,576	)



	Condensed Consolidating Statement of Cash Flows
	Year ended December 31, 2017
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net cash (used in) provided by operating activities	$	(36,575	)	$	101,826		$	83,871		$	—		$	149,122
Investing activities:
Capital expenditures	—			(13,215		)	(8,292		)	—			(21,507		)
Proceeds from sale of assets	—			—			6,766			—			6,766
Dividends received from subsidiaries	—			307,784			—			(307,784		)	—
Advances to subsidiaries and other	—			(383,686		)	26,912			366,220			9,446
Net cash (used in) provided by investing activities	—			(89,117		)	25,386			58,436			(5,295		)
Financing activities:
Proceeds from issuance of debt	143,367			775,926			1,144,848			(383,686		)	1,680,455
Repayments of debt	—			(785,392		)	(952,740		)	17,466			(1,720,666		)
Equity activities	(106,792		)	—			—			—			(106,792		)
Dividends paid by subsidiaries	—			—			(307,784		)	307,784			—
Other	—			5,807			—			—			5,807
Net cash provided by (used in) financing activities	36,575			(3,659		)	(115,676		)	(58,436		)	(141,196		)
Effect of exchange rate changes on cash and cash equivalents	—			—			5,191			—			5,191
Net change in cash and cash equivalents	—			9,050			(1,228		)	—			7,822
Cash and cash equivalents at the beginning of period	—			41,552			68,579			—			110,131
Cash and cash equivalents at the end of period	$	—		$	50,602		$	67,351		$	—		$	117,953



	Condensed Consolidating Statement of Cash Flows
	Year ended December 31, 2016
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net cash provided by (used in) operating activities	$	95,388		$	(243,476	)	$	448,323		$	—		$	300,235
Investing activities:
Capital expenditures	—			(12,482		)	(5,475		)	—			(17,957		)
Acquisition payments, net of cash acquired	—			(50,890		)	—			—			(50,890		)
Proceeds from sale of assets	—			—			8,361			—			8,361
Dividends received from subsidiaries	—			82,912			—			(82,912		)	—
Advances to subsidiaries and other	—			(297,259		)	(337,344		)	624,603			(10,000		)
Net cash used in investing activities	—			(277,719		)	(334,458		)	541,691			(70,486		)
Financing activities:
Proceeds from issuance of debt	252,246			1,566,864			672,345			(297,259		)	2,194,196
Repayments of debt	(344,804		)	(1,030,520		)	(752,401		)	(327,344		)	(2,455,069		)
Equity activities	(2,830		)	—			—			—			(2,830		)
Dividends paid by subsidiaries	—			—			(82,912		)	82,912			—
Other	—			(12,560		)	—			—			(12,560		)
Net cash (used in) provided by financing activities	(95,388		)	523,784			(162,968		)	(541,691		)	(276,263		)
Effect of exchange rate changes on cash and cash equivalents	—			—			(3,634		)	—			(3,634		)
Net change in cash and cash equivalents	—			2,589			(52,737		)	—			(50,148		)
Cash and cash equivalents at the beginning of period	—			38,963			121,316			—			160,279
Cash and cash equivalents at the end of period	$	—		$	41,552		$	68,579		$	—		$	110,131



	Condensed Consolidating Statement of Cash Flows
	Year ended December 31, 2015
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net cash provided by operating activities	$	3,531		$	214,037		$	65,481		$	—		$	283,049
Investing activities:
Capital expenditures	—			(15,266		)	(6,392		)	—			(21,658		)
Acquisition payments, net of cash acquired	—			(151,595		)	—			—			(151,595		)
Proceeds from sale of assets	—			—			3,023			—			3,023
Dividends received from subsidiaries	—			114,101			—			(114,101		)	—
Advances to subsidiaries and other	—			(197,345		)	17,461			179,884			—
Net cash (used in) provided by investing activities	—			(250,105		)	14,092			65,783			(170,230		)
Financing activities:
Proceeds from issuance of debt	150,705			1,224,596			452,655			(197,345		)	1,630,611
Repayments of debt	—			(1,175,056		)	(379,578		)	17,461			(1,537,173		)
Equity activities	(154,236		)	—			—			—			(154,236		)
Dividends paid by subsidiaries	—			—			(114,101		)	114,101			—
Other	—			(7,017		)	—			—			(7,017		)
Net cash provided by (used in) financing activities	(3,531		)	42,523			(41,024		)	(65,783		)	(67,815		)
Effect of exchange rate changes on cash and cash equivalents	—			—			(13,044		)	—			(13,044		)
Net change in cash and cash equivalents	—			6,455			25,505			—			31,960
Cash and cash equivalents at the beginning of period	—			32,508			95,811			—			128,319
Cash and cash equivalents at the end of period	$	—		$	38,963		$	121,316		$	—		$	160,279

Revisions

As described in Note 2, the Consolidated Balance Sheet at December 31, 2016 has been revised to correct certain financial statement line items.

SELECTED QUARTERLY FINANCIAL DATA

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

SELECTED QUARTERLY FINANCIAL DATA

16. SELECTED QUARTERLY FINANCIAL DATA (unaudited)

The following table sets forth selected quarterly financial data for the years ended December 31, 2017 and 2016:



	First Quarter		Second Quarter		Third Quarter			Fourth Quarter
2017
Net Sales	$	1,772,591	$	1,909,624	$	2,000,159		$	1,996,647
Cost of goods sold (excluding depreciation and amortization)	1,422,573		1,543,510		1,614,814			1,613,469
Income from operations	67,089		83,105		89,250			81,446
Income before income taxes	50,368		66,289		71,939			63,844
Net income⁽¹⁾	37,800		49,535		53,576			22,222
Net income attributable to WESCO International⁽¹⁾	37,729		49,510		53,675			22,546
Basic earnings per share attributable to WESCO International ^{(1) (2)}	0.77		1.03		1.13			0.48
Diluted earnings per share attributable to WESCO International ^{(1) (3)}	0.76		1.02		1.12			0.47

2016
Net Sales	$	1,775,961	$	1,911,582	$	1,855,212		$	1,793,262
Cost of goods sold (excluding depreciation and amortization)	1,420,793		1,532,113		1,490,173			1,444,735
Income from operations	69,508		87,987		92,555			82,009
Income (loss) before income taxes	50,679		68,535		(52,170		)	64,507
Net income (loss)	34,534		49,852		(31,021		)	47,755
Net income (loss) attributable to WESCO International	36,053		49,798		(31,611		)	47,348
Basic earnings (loss) per share attributable to WESCO International ^{(2) (4)}	0.85		1.18		(0.73		)	0.97
Diluted earnings (loss) per share attributable to WESCO International ^{(3) (4)}	0.77		1.02		(0.73		)	0.96


⁽¹⁾	As described in Note 9, net income and net income attributable to WESCO International include provisional discrete income tax expense of $26.4 million resulting from the application of the TCJA, which affected basic and diluted earnings per share attributable to WESCO International in the fourth quarter of 2017.


⁽²⁾	Earnings per share (EPS) in each quarter is computed using the weighted-average number of shares outstanding during that quarter while EPS for the full year is computed by using the weighted-average number of shares outstanding during the year. Thus, the sum of the four quarters’ EPS may not equal the full-year EPS.


⁽³⁾	Diluted EPS in each quarter is computed using the weighted-average number of shares outstanding and common share equivalents during that quarter while Diluted EPS for the full year is computed by using the weighted-average number of shares outstanding and common share equivalents during the year. Thus, the sum of the four quarters’ Diluted EPS may not equal the full-year Diluted EPS.

⁽⁴⁾

On September 15, 2016, the Company completed the redemption of its 2029 Debentures. The redemption resulted in a non-cash charge of $123.9 million and consequently a net loss attributable to WESCO International for the three months ended September 30, 2016. Accordingly, dilutive shares were not included in the calculation of diluted loss per share for the three months ended September 30, 2016 because their effect was antidilutive. As described in Note 9, net income and net income attributable to WESCO International include provisional discrete income tax expense of $26.4 million resulting from the application of the TCJA, which affected basic and diluted earnings per share attributable to WESCO International in the fourth quarter of 2017.

Schedule II - Valuation and Qualifying Accounts

12 Months Ended

Dec. 31, 2016

Valuation and Qualifying Accounts [Abstract]

Schedule II - Valuation and Qualifying Accounts

Schedule II—Valuation and Qualifying Accounts



	Balance at Beginning		Charged to	Charged to Other			Balance at
	of Period		Expense	Accounts⁽¹⁾	Deductions⁽²⁾		End of Period
Allowance for doubtful accounts	(In thousands)
Year ended December 31, 2017	$	22,007	8,466	—	(9,160	)	$	21,313
Year ended December 31, 2016	22,587		5,888	21	(6,489	)	22,007
Year ended December 31, 2015	21,084		6,099	1,305	(5,901	)	22,587

_________________________


⁽¹⁾	Represents allowance for doubtful accounts in connection with certain acquisitions and divestitures.


⁽²⁾	Includes a reduction in the allowance for doubtful accounts due to write-off of accounts receivable.

ACCOUNTING POLICIES (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Basis of Consolidation

Use of Estimates

Revenue Recognition

Selling, General and Administrative Expenses

WESCO includes warehousing, purchasing, branch operations, information services, and marketing and selling expenses in this category, as well as other types of general and administrative costs.

Supplier Volume Rebates

Shipping and Handling Costs and Fees

Cash Equivalents

Cash equivalents are defined as highly liquid investments with original maturities of 90 days or less when purchased.

Allowance for Doubtful Accounts

Inventories

Other Assets

Debt Issuance Costs

Property, Buildings and Equipment

Goodwill and Intangible Assets

Goodwill and Indefinite-Lived Intangible Assets

Definite Lived Intangible Assets

Insurance Programs

Income Taxes

Convertible Debentures

Foreign Currency

Defined Benefit Pension Plan

Stock-Based Compensation

Treasury Stock

Fair Value of Financial Instruments


•	Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities that are accessible at the measurement date.

•


•	Level 3 inputs are unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions.

Environmental Expenditures

New Accounting Pronouncements, Policy [Policy Text Block]

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Revenue Recognition, Rebates [Policy Text Block]

Supplier Volume Rebates

Accounting Changes and Error Corrections [Text Block]

Revision of Prior Period Financial Statements

The following table presents the effects on the financial statement line items that were revised:



	December 31, 2016
	As Reported			Adjustment			As Revised
	(In thousands)
Income taxes receivable	$	72,881		$	(67,156	)	$	5,725
Prepaid expenses and other current assets	48,583			(2,223		)	46,360
Total current assets	2,172,457			(69,379		)	2,103,078
Goodwill	1,720,714			10,236			1,730,950
Total assets	4,490,984			(59,143		)	4,431,841
Income taxes payable	32,879			(22,998		)	9,881
Total current liabilities	896,797			(22,998		)	873,799
Deferred income taxes	158,009			10,236			168,245
Total liabilities	2,480,972			(12,762		)	2,468,210
Retained earnings ⁽¹⁾	1,956,532			(41,775		)	1,914,757
Accumulated other comprehensive loss ⁽¹⁾	(387,365		)	(4,606		)	(391,971		)
Total WESCO International, Inc. stockholders' equity	2,013,281			(46,381		)	1,966,900
Total stockholders' equity	2,010,012			(46,381		)	1,963,631
Total liabilities and stockholders' equity	4,490,984			(59,143		)	4,431,841


⁽¹⁾	These financial statement line items have been revised as of December 31, 2015 and 2014 in the Consolidated Statements of Stockholders' Equity for the years ended December 31, 2017, 2016 and 2015.

ACCOUNTING POLICIES ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Schedule of Quantifying Prior Year Misstatements Corrected in Current Year Financial Statements [Table Text Block]

The following table presents the effects on the financial statement line items that were revised:



	December 31, 2016
	As Reported			Adjustment			As Revised
	(In thousands)
Income taxes receivable	$	72,881		$	(67,156	)	$	5,725
Prepaid expenses and other current assets	48,583			(2,223		)	46,360
Total current assets	2,172,457			(69,379		)	2,103,078
Goodwill	1,720,714			10,236			1,730,950
Total assets	4,490,984			(59,143		)	4,431,841
Income taxes payable	32,879			(22,998		)	9,881
Total current liabilities	896,797			(22,998		)	873,799
Deferred income taxes	158,009			10,236			168,245
Total liabilities	2,480,972			(12,762		)	2,468,210
Retained earnings ⁽¹⁾	1,956,532			(41,775		)	1,914,757
Accumulated other comprehensive loss ⁽¹⁾	(387,365		)	(4,606		)	(391,971		)
Total WESCO International, Inc. stockholders' equity	2,013,281			(46,381		)	1,966,900
Total stockholders' equity	2,010,012			(46,381		)	1,963,631
Total liabilities and stockholders' equity	4,490,984			(59,143		)	4,431,841


⁽¹⁾	These financial statement line items have been revised as of December 31, 2015 and 2014 in the Consolidated Statements of Stockholders' Equity for the years ended December 31, 2017, 2016 and 2015.

GOODWILL AND INTANGIBLE ASSETS (Tables)

12 Months Ended

Dec. 31, 2017

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of Goodwill

The following table sets forth the changes in the carrying value of goodwill:



	Year Ended December 31,
	2017		2016
	(In thousands)
Beginning balance January 1	$	1,730,950	$	1,681,662
Foreign currency exchange rate changes	40,927		21,434
Adjustments to goodwill for acquisitions ⁽¹⁾	—		17,618
Prior period revision (Note 2)	—		10,236
Ending balance December 31	$	1,771,877	$	1,730,950


⁽¹⁾	For the year ended December 31, 2016, adjustments relate to goodwill resulting from the preliminary allocation of the purchase price for Atlanta Electrical Distributors, LLC to the respective assets acquired and liabilities assumed, partially offset by an adjustment to goodwill related to deferred income taxes.

Schedule of Intangible Assets by Major Class

The components of intangible assets are as follows:



		December 31, 2017							December 31, 2016
	Life	Gross Carrying Amount ⁽¹⁾		Accumulated Amortization ⁽¹⁾			Net Carrying Amount		Gross Carrying Amount ⁽¹⁾		Accumulated Amortization ⁽¹⁾			Net Carrying Amount
		(In thousands)
Intangible assets:
Trademarks	Indefinite	$	100,249	$	—		$	100,249	$	96,962	$	—		$	96,962
Trademarks	4-15	25,118		(5,516		)	19,602		25,098		(3,426		)	21,672
Non-compete agreements	2-7	196		(102		)	94		196		(63		)	133
Customer relationships	2-20	377,270		(161,711		)	215,559		362,637		(126,835		)	235,802
Distribution agreements	10-19	39,515		(22,200		)	17,315		38,972		(19,295		)	19,677
Patents	10	48,310		(34,025		)	14,285		48,310		(29,194		)	19,116
		$	590,658	$	(223,554	)	$	367,104	$	572,175	$	(178,813	)	$	393,362


⁽¹⁾	Excludes the original cost and accumulated amortization of fully-amortized intangible assets.

Schedule of Expected Amortization Expense

The following table sets forth the estimated amortization expense for intangible assets for the next five years:



For the year ending December 31,	(In thousands)
2018	$	37,848
2019	36,549
2020	34,621
2021	26,930
2022	24,350

ACQUISITIONS (Tables)

12 Months Ended

Dec. 31, 2017

Business Acquisition [Line Items]

Schedule of Business Acquisitions, by Acquisition

The following table sets forth the consideration paid for acquisitions:



	Year Ended December 31,
	2017		2016			2015
	(In thousands)
Fair value of assets acquired	$	—	$	76,980		$	192,099
Fair value of liabilities assumed	—		25,058			39,836
Cash paid for acquisitions	$	—	$	51,922		$	152,263
Supplemental cash flow disclosure related to acquisitions:
Cash paid for acquisitions	$	—	$	51,922		$	152,263
Less: cash acquired	—		(1,032		)	(668		)
Cash paid for acquisitions, net of cash acquired	$	—	$	50,890		$	151,595

PROPERTY, BUILDINGS AND EQUIPMENT (Tables)

12 Months Ended

Dec. 31, 2017

Property, Plant and Equipment [Abstract]

Property, Plant and Equipment

The following table sets forth the components of property, buildings and equipment:



	As of December 31,
	2017			2016
	(In thousands)
Buildings and leasehold improvements	$	117,894		$	117,461
Furniture, fixtures and equipment	183,801			178,183
Software costs	103,842			93,040
	405,537			388,684
Accumulated depreciation and amortization	(278,455		)	(259,126		)
	127,082			129,558
Land	25,814			24,653
Construction in progress	3,549			3,396
	$	156,445		$	157,607

DEBT (Tables)

12 Months Ended

Dec. 31, 2017

Debt Instrument [Line Items]

Schedule of Debt

The following table sets forth WESCO’s outstanding indebtedness:



	As of December 31,
	2017			2016
	(In thousands)
International lines of credit	$	34,075		$	20,920
Term Loan Facility, less debt discount of $513 and $770 in 2017 and 2016, respectively	84,237			143,980
Accounts Receivable Securitization Facility	380,000			380,000
Revolving Credit Facility	12,000			4,000
5.375% Senior Notes due 2021	500,000			500,000
5.375% Senior Notes due 2024	350,000			350,000
Capital leases	1,959			2,881
Total debt	1,362,271			1,401,781
Less unamortized debt issuance costs	(13,711		)	(16,508		)
Less short-term debt and current portion of long-term debt	(35,299		)	(22,138		)
Total long-term debt	$	1,313,261		$	1,363,135

Schedule of Maturities of Long-term Debt

The following table sets forth the aggregate principal repayment requirements for all indebtedness for the next five years and thereafter, as of December 31, 2017:



	(In thousands)
2018	$	35,299
2019	85,246
2020	392,239
2021	500,000
2022	—
Thereafter	350,000
Total payments on debt	1,362,784
Debt discount	(513		)
Total debt	$	1,362,271

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Components of Income Tax Expense

The following table sets forth the components of the provision (benefit) for income taxes:



	Year Ended December 31,
	2017			2016			2015
				(In thousands)
Current income taxes:
Federal ⁽¹⁾	$	122,170		$	65,614		$	45,812
State	2,259			6,489			4,565
Foreign	15,274			3,502			2,309
Total current income taxes	139,703			75,605			52,686
Deferred income taxes:
Federal	(48,060		)	(42,835		)	29,593
State	4,508			(2,938		)	3,767
Foreign	(6,844		)	599			9,491
Total deferred income taxes	(50,396		)	(45,174		)	42,851
Provision for income taxes	$	89,307		$	30,431		$	95,537


⁽¹⁾	Income tax (expense) benefit related to stock-based awards and other equity instruments recorded directly to additional paid in capital totaled $(0.1) million and $1.6 million in 2016 and 2015, respectively. Due to the adoption of ASU 2016-09, as described in Note 2, there was no income tax (expense) benefit recorded to additional paid in capital for stock-based awards in 2017.

Schedule of Income before Income Tax, Domestic and Foreign

The following table sets forth the components of income before income taxes by jurisdiction:



	Year Ended December 31,
	2017		2016		2015
			(In thousands)
United States	$	180,957	$	80,881	$	288,881
Foreign	71,483		50,670		15,029
Income before income taxes	$	252,440	$	131,551	$	303,910

Schedule of Effective Income Tax Rate Reconciliation

The following table sets forth the reconciliation between the federal statutory income tax rate and the effective tax rate:



	Year Ended December 31,
	2017		2016		2015
Federal statutory rate	35.0	%	35.0	%	35.0	%
State income taxes, net of federal income tax benefit	1.4		1.0		2.2
Deemed repatriation of undistributed foreign earnings	32.8		—		—
Deferred income tax remeasurement	(22.4	)	—		—
Tax effect of intercompany financing	(10.5	)	(19.9	)	(8.8	)
Foreign tax rate differences	(1.3	)	(0.4	)	(1.1	)
Valuation allowance against deferred tax assets	0.4		1.1		—
Nondeductible expenses	(0.1	)	1.6		1.2
Adjustment related to uncertain tax positions	—		3.7		2.7
Other	0.1		1.0		0.2
Effective tax rate	35.4	%	23.1	%	31.4	%

Schedule of Deferred Tax Assets and Liabilities

The following table sets forth deferred tax assets and liabilities:



	As of December 31,
	2017					2016
				(In thousands)
	Assets			Liabilities		Assets			Liabilities
Accounts receivable	$	3,496		$	—	$	3,484		$	—
Inventories	—			3,181		—			4,001
Depreciation of property, buildings and equipment	—			13,283		—			11,487
Amortization of intangible assets ⁽¹⁾	—			159,107		—			237,015
Employee benefits	14,835			—		18,577			—
Stock-based compensation ⁽²⁾	16,341			—		23,844			—
Advance payments	8,456			—		22,056			—
Foreign tax credits	—			—		15,698			—
Tax loss carryforwards	19,128			—		18,440			—
Other	11,850			8,672		7,175			7,783
Deferred income taxes before valuation allowance	74,106			184,243		109,274			260,286
Valuation allowance	(2,518		)	—		(1,430		)	—
Total deferred income taxes ⁽¹⁾	$	71,588		$	184,243	$	107,844		$	260,286


⁽¹⁾	As described in Note 2, the Consolidated Balance Sheet at December 31, 2016 was revised to correct certain financial statement line items, including deferred income taxes.


⁽²⁾	The Company does not expect the realizability of the deferred tax asset related to stock-based compensation to be materially impacted by the TCJA's expansion on the limitation of deductions for excessive employee compensation.

Summary of Income Tax Examinations



United States — Federal		2004 and forward
United States — Material States		2013 and forward
Canada		2004 and forward



United States — Federal		2004 and forward
United States — Material States		2013 and forward
Canada		2004 and forward

Summary of Income Tax Contingencies

The following table sets forth the reconciliation of gross unrecognized tax benefits:



	As of December 31,
	2017			2016			2015
	(In thousands)
Beginning balance January 1	$	6,181		$	5,436		$	20,033
Additions based on tax positions related to the current year	—			—			46
Additions for tax positions of prior years	—			3,298			402
Reductions for tax positions of prior years	(155		)	(21		)	(378		)
Settlements	(1,025		)	(1,921		)	(9,638		)
Lapse in statute of limitations	(755		)	(728		)	(1,497		)
Foreign currency exchange rate changes	102			117			(3,532		)
Ending balance December 31	$	4,348		$	6,181		$	5,436

EARNINGS PER SHARE (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Schedule of Earnings Per Share, Basic and Diluted



	Year Ended December 31,
	2017		2016		2015
(In thousands, except per share data)
Net income attributable to WESCO International	$	163,460	$	101,588	$	210,687
Weighted-average common shares outstanding used in computing basic earnings per share	47,849		44,116		43,433
Common shares issuable upon exercise of dilutive equity awards	512		543		626
Common shares issuable from contingently convertible debentures (see below for basis of calculation)	—		3,674		6,314
Weighted-average common shares outstanding and common share equivalents used in computing diluted earnings per share	48,361		48,333		50,373
Earnings per share attributable to WESCO International
Basic	$	3.42	$	2.30	$	4.85
Diluted	$	3.38	$	2.10	$	4.18

EMPLOYEE BENEFIT PLANS - (Tables)

12 Months Ended

Dec. 31, 2017

Retirement Benefits [Abstract]

Schedule of Net Funded Status [Table Text Block]

The following tables present the changes in benefit obligations, plan assets and funded status for the pension plans and the components of net periodic pension cost.



	Year Ended December 31,
(In thousands)	2017			2016
Accumulated Benefit Obligation (ABO) at December 31	$	92,375		$	75,666

Change in Projected Benefit Obligation (PBO)
PBO at beginning of year	$	96,160		$	87,186
Service cost	4,328			3,845
Interest cost	3,912			3,856
Participant contributions	735			709
Actuarial loss, including assumption changes	10,906			2,172
Benefits paid	(3,005		)	(4,404		)
Foreign currency exchange rate changes	7,283			2,796
PBO at end of year	$	120,319		$	96,160

Change in Plan Assets
Fair value of plan assets at beginning of year	$	84,753		$	79,185
Actual return on plan assets	7,875			4,115
Participant contributions	735			709
Employer contributions	368			1,956
Benefits paid	(3,005		)	(4,404		)
Foreign currency exchange rate changes	6,456			3,192
Fair value of plan assets at end of year	$	97,182		$	84,753

Funded Status	$	(23,137	)	$	(11,407	)

Amounts Recognized in the Consolidated Balance Sheets
Current liabilities	$	(395	)	$	(364	)
Noncurrent liabilities	(22,742		)	(11,043		)
Net amount recognized	$	(23,137	)	$	(11,407	)

Amounts Recognized in Accumulated Other Comprehensive Income (Loss)
Net actuarial loss (gain)	$	2,508		$	(6,234	)
Total amount recognized, before tax effect	$	2,508		$	(6,234	)

Schedule of Net Benefit Costs [Table Text Block]



	Year Ended December 31,
	2017			2016			2015
	(In thousands)
Components of Net Periodic Pension Cost
Service cost	$	4,328		$	3,845		$	4,537
Interest cost	3,912			3,856			4,012
Expected return on plan assets	(5,562		)	(5,328		)	(5,260		)
Recognized actuarial gain	(149		)	(31		)	(15		)
Net periodic pension cost	$	2,529		$	2,342		$	3,274

Other Changes in Plan Assets and PBO Recognized in Accumulated Other Comprehensive Income (Loss)
Net actuarial loss (gain)	$	8,593		$	2,756		$	(6,208	)
Amortization of unrecognized net actuarial gain	149			31			15
Total amount recognized, before tax effect	8,742			2,787			(6,193		)
Tax effect	(2,361		)	(302		)	1,661
Total amount recognized, after tax effect	$	6,381		$	2,485		$	(4,532	)

Total recognized in net periodic pension cost and accumulated other comprehensive income (loss)	$	8,910		$	4,827		$	(1,258	)

Schedule of Allocation of Plan Assets [Table Text Block]

The asset mix is reviewed and rebalanced to target on an annual basis.



Asset Category	Target %
Canadian equities	12.5	%
Non-Canadian equities	27.5	%
Total equities	40	%
Fixed income investments	45	%
Other investments	15	%

The Plan's weighted asset allocations by asset category are as follows:



	December 31
	2017		2016
Asset Category
Pooled Funds:
Canadian equities	11.5	%	11.7	%
U.S. equities	4.6	%	4.6	%
Non-North American equities	20.8	%	21.6	%
Fixed income investments	41.4	%	43.4	%
Other	21.7	%	18.7	%
Total	100.0	%	100.0	%

Fair Value, Assets Measured on Recurring Basis [Table Text Block]

The following tables set forth the fair value of the Plan's assets by asset category:



	December 31, 2017
(In thousands)	Level 1		Level 2		Level 3		NAV⁽¹⁾		Total
Pooled Funds:
Canadian equities	$	—	$	—	$	—	$	11,211	$	11,211
U.S. equities	—		—		—		4,436		4,436
Non-North American equities	—		—		—		20,207		20,207
Fixed income investments	—		—		—		40,193		40,193
Other	3,996		—		—		17,139		21,135
Total investments	$	3,996	$	—	$	—	$	93,186	$	97,182



	December 31, 2016
(In thousands)	Level 1		Level 2		Level 3		NAV⁽¹⁾		Total
Pooled Funds:
Canadian equities	$	—	$	—	$	—	$	9,916	$	9,916
U.S. equities	—		—		—		3,881		3,881
Non-North American equities	—		—		—		18,296		18,296
Fixed income investments	—		—		—		36,677		36,677
Other	235		—		—		15,748		15,983
Total investments	$	235	$	—	$	—	$	84,518	$	84,753


⁽¹⁾	As described above, investments measured at fair value using the NAV per share practical expedient have not been classified in the fair value hierarchy. The amounts presented in the tables are intended to reconcile the fair value hierarchy to the total fair value of plan assets.

Pension and Other Postretirement Plans, Pensions, Policy [Policy Text Block]

11. EMPLOYEE BENEFIT PLANS

Defined Contribution Plans

A majority of WESCO’s employees are covered by defined contribution retirement savings plans for their services rendered subsequent to WESCO’s formation. WESCO also offers a deferred compensation plan for select individuals. For U.S. participants, WESCO matches contributions made by employees at an amount equal to 50% of participants' total monthly contributions up to a maximum of 6% of eligible compensation. For Canadian participants, WESCO makes contributions in amounts ranging from 3% to 5% of participants' eligible compensation based on years of continuous service. WESCO may also make, subject to the Board of Directors' approval, a discretionary contribution to the defined contribution retirement savings plan covering U.S. participants if certain predetermined profit levels are attained. A discretionary employer contribution charge of $10.0 million was incurred in 2017. In 2016 and 2015, there were no charges for discretionary employer contributions. For the years ended December 31, 2017, 2016 and 2015, WESCO incurred charges of $31.3 million, $18.5 million, and $18.1 million, respectively, for all such plans. Contributions are made in cash to employee retirement savings plan accounts. The deferred compensation plan is an unfunded plan. As of December 31, 2017 and 2016, the Company's obligation under the deferred compensation plan was $24.3 million and $21.7 million, respectively. Employees have the option to transfer balances allocated to their accounts in the defined contribution retirement savings plan and the deferred compensation plan into any of the available investment options.

Defined Benefit Plans

The Company sponsors a contributory defined benefit plan (the "Plan") covering substantially all Canadian employees of EECOL. The Plan provides retirement benefits based on earnings and credited service, and participants contribute 2% of their earnings to the Plan. Participants become 100% vested after two years of continuous service or, if earlier, at the participant's normal retirement age.

The Company also sponsors a Supplemental Executive Retirement Plan (the "SERP"), which provides additional pension benefits to certain executives of EECOL based on earnings, and credited service. Effective January 1, 2013, the SERP was closed to new participants and existing participants became 100% vested. SERP participants continue to contribute 4% of their earnings to the Plan.

The following tables present the changes in benefit obligations, plan assets and funded status for the pension plans and the components of net periodic pension cost.



	Year Ended December 31,
(In thousands)	2017			2016
Accumulated Benefit Obligation (ABO) at December 31	$	92,375		$	75,666

Change in Projected Benefit Obligation (PBO)
PBO at beginning of year	$	96,160		$	87,186
Service cost	4,328			3,845
Interest cost	3,912			3,856
Participant contributions	735			709
Actuarial loss, including assumption changes	10,906			2,172
Benefits paid	(3,005		)	(4,404		)
Foreign currency exchange rate changes	7,283			2,796
PBO at end of year	$	120,319		$	96,160

Change in Plan Assets
Fair value of plan assets at beginning of year	$	84,753		$	79,185
Actual return on plan assets	7,875			4,115
Participant contributions	735			709
Employer contributions	368			1,956
Benefits paid	(3,005		)	(4,404		)
Foreign currency exchange rate changes	6,456			3,192
Fair value of plan assets at end of year	$	97,182		$	84,753

Funded Status	$	(23,137	)	$	(11,407	)

Amounts Recognized in the Consolidated Balance Sheets
Current liabilities	$	(395	)	$	(364	)
Noncurrent liabilities	(22,742		)	(11,043		)
Net amount recognized	$	(23,137	)	$	(11,407	)

Amounts Recognized in Accumulated Other Comprehensive Income (Loss)
Net actuarial loss (gain)	$	2,508		$	(6,234	)
Total amount recognized, before tax effect	$	2,508		$	(6,234	)



	Year Ended December 31,
	2017			2016			2015
	(In thousands)
Components of Net Periodic Pension Cost
Service cost	$	4,328		$	3,845		$	4,537
Interest cost	3,912			3,856			4,012
Expected return on plan assets	(5,562		)	(5,328		)	(5,260		)
Recognized actuarial gain	(149		)	(31		)	(15		)
Net periodic pension cost	$	2,529		$	2,342		$	3,274

Other Changes in Plan Assets and PBO Recognized in Accumulated Other Comprehensive Income (Loss)
Net actuarial loss (gain)	$	8,593		$	2,756		$	(6,208	)
Amortization of unrecognized net actuarial gain	149			31			15
Total amount recognized, before tax effect	8,742			2,787			(6,193		)
Tax effect	(2,361		)	(302		)	1,661
Total amount recognized, after tax effect	$	6,381		$	2,485		$	(4,532	)

Total recognized in net periodic pension cost and accumulated other comprehensive income (loss)	$	8,910		$	4,827		$	(1,258	)

The following weighted-average actuarial assumptions were used to determine benefit obligations at December 31:



	2017				2016
	Pension Plan		SERP		Pension Plan		SERP
Discount rate	3.5	%	3.5	%	3.9	%	3.9	%
Rate of compensation increase	3.8	%	3.8	%	3.8	%	3.8	%

The following weighted-average actuarial assumptions were used to determine net periodic pension costs at January 1:



	Year Ended December 31,
	2017				2016				2015
	Pension Plan		SERP		Pension Plan		SERP		Pension Plan		SERP
Discount rate	3.9	%	3.9	%	4.2	%	4.2	%	4.1	%	4.1	%
Expected long-term return on assets	6.4	%	n/a		6.4	%	n/a		6.4	%	n/a
Rate of compensation increase	3.8	%	3.8	%	4.0	%	4.0	%	4.0	%	4.0	%

The following benefit payments, which reflect expected future service, are expected to be paid:



Years ending December 31	(In thousands)
2018	$	3,092
2019	3,131
2020	3,215
2021	3,382
2022	3,488
2023 to 2027	21,776

The Company expects to contribute approximately $0.4 million to the SERP in 2018.

The Plan's weighted asset allocations by asset category are as follows:



	December 31
	2017		2016
Asset Category
Pooled Funds:
Canadian equities	11.5	%	11.7	%
U.S. equities	4.6	%	4.6	%
Non-North American equities	20.8	%	21.6	%
Fixed income investments	41.4	%	43.4	%
Other	21.7	%	18.7	%
Total	100.0	%	100.0	%

The Plan's long-term overall objective is to maintain benefits at their current level without affecting the cost of maintaining the Plan, assuming that the demographic make-up of the group of members remains the same.

The primary investment objective, in support of the overall objective, is to earn the highest rate of return possible for the Plan, while keeping risk at acceptable levels. The long-term return objective of the Plan is to achieve a minimum annualized rate of return in excess of the actuarial requirements. This translates into a required return of 3.0% above inflation, net of investment management fees. The return objective is consistent with the overall investment risk level that the Plan assumes in order to meet the pension obligations of the Plan. To achieve this long term investment objective, the Plan has adopted an asset mix that has a combination of primarily equity and fixed income investments. Risk is controlled by investing in a well-diversified portfolio of asset classes. A benchmark portfolio is established based on the expected returns for each asset class available. The investment of the Plan's assets in accordance with the benchmark portfolio should enable the Plan to not only attain, but also exceed the minimum overall objective.

The following table presents the target asset mix based on market value for each investment category within which the investment managers must invest the Plan's assets. The asset mix is reviewed and rebalanced to target on an annual basis.



Asset Category	Target %
Canadian equities	12.5	%
Non-Canadian equities	27.5	%
Total equities	40	%
Fixed income investments	45	%
Other investments	15	%

The Plan's assets are measured at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities are classified in the fair value hierarchy based on the lowest level of any input that is significant to the measurement of fair value. Investments for which fair value is measured using the net asset value (NAV) per share practical expedient are not classified in the fair value hierarchy. The following describes the valuation methodologies used to measure the fair value of the Plan's assets.

Pooled Equity Investments. These investments consist of the Plan's share of segregated funds that primarily invest in equity securities. The funds are valued at the net asset value of shares held in the underlying funds.

Pooled Fixed Income Investments. These investments consist of the Plan's share of a segregated fund that primarily invests in Canadian issued bonds and debentures and is valued at the net asset value of shares held in the underlying funds.

Other Investments. These investments consist of cash and cash equivalents, a money market fund and diversified growth funds. The diversified growth funds invest in a broad range of asset classes, including equities, bonds, infrastructure, property, commodities and absolute return strategies. These investments are valued at the net asset value of shares held in the underlying funds.

The fair value methods described above may not be indicative of net realizable value or reflective of future fair values. Additionally, while the Company believes the valuation methods are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date.

The following tables set forth the fair value of the Plan's assets by asset category:



	December 31, 2017
(In thousands)	Level 1		Level 2		Level 3		NAV⁽¹⁾		Total
Pooled Funds:
Canadian equities	$	—	$	—	$	—	$	11,211	$	11,211
U.S. equities	—		—		—		4,436		4,436
Non-North American equities	—		—		—		20,207		20,207
Fixed income investments	—		—		—		40,193		40,193
Other	3,996		—		—		17,139		21,135
Total investments	$	3,996	$	—	$	—	$	93,186	$	97,182



	December 31, 2016
(In thousands)	Level 1		Level 2		Level 3		NAV⁽¹⁾		Total
Pooled Funds:
Canadian equities	$	—	$	—	$	—	$	9,916	$	9,916
U.S. equities	—		—		—		3,881		3,881
Non-North American equities	—		—		—		18,296		18,296
Fixed income investments	—		—		—		36,677		36,677
Other	235		—		—		15,748		15,983
Total investments	$	235	$	—	$	—	$	84,518	$	84,753


⁽¹⁾	As described above, investments measured at fair value using the NAV per share practical expedient have not been classified in the fair value hierarchy. The amounts presented in the tables are intended to reconcile the fair value hierarchy to the total fair value of plan assets.

Schedule of Assumptions Used [Table Text Block]

The following weighted-average actuarial assumptions were used to determine benefit obligations at December 31:



	2017				2016
	Pension Plan		SERP		Pension Plan		SERP
Discount rate	3.5	%	3.5	%	3.9	%	3.9	%
Rate of compensation increase	3.8	%	3.8	%	3.8	%	3.8	%

The following weighted-average actuarial assumptions were used to determine net periodic pension costs at January 1:



	Year Ended December 31,
	2017				2016				2015
	Pension Plan		SERP		Pension Plan		SERP		Pension Plan		SERP
Discount rate	3.9	%	3.9	%	4.2	%	4.2	%	4.1	%	4.1	%
Expected long-term return on assets	6.4	%	n/a		6.4	%	n/a		6.4	%	n/a
Rate of compensation increase	3.8	%	3.8	%	4.0	%	4.0	%	4.0	%	4.0	%

Schedule of Expected Benefit Payments [Table Text Block]

The following benefit payments, which reflect expected future service, are expected to be paid:



Years ending December 31	(In thousands)
2018	$	3,092
2019	3,131
2020	3,215
2021	3,382
2022	3,488
2023 to 2027	21,776

STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Share-based Compensation, Stock Options and Stock Appreciation Rights Award Activity

The following table sets forth a summary of stock-settled stock appreciation rights and related information for the years indicated:



	2017							2016				2015
	Awards		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Life	Aggregate Intrinsic Value (In thousands)		Awards		Weighted-Average Exercise Price		Awards		Weighted-Average Exercise Price
Beginning of year	2,439,487		$	52.62				2,567,021		$	54.47	2,480,745		$	50.91
Granted	455,807		71.21					709,999		42.63		394,182		69.54
Exercised	(495,181	)	42.19					(526,818	)	41.54		(232,542	)	35.80
Canceled	(161,506	)	66.06					(310,715	)	63.71		(75,364	)	73.59
End of year	2,238,607		57.75		6.2	$	28,791	2,439,487		52.62		2,567,021		54.47
Exercisable at end of year	1,331,580		$	56.96	4.6	$	18,801	1,549,350		$	53.35	2,034,263		$	49.36

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

WESCO granted the following stock-settled stock appreciation rights at the following weighted-average assumptions:



	2017	2016	2015
Stock-settled stock appreciation rights granted	455,807	709,999	394,182
Risk free interest rate	1.9%	1.2%	1.6%
Expected life (in years)	5	5	5
Expected volatility	29%	32%	32%

Schedule of Share-based Compensation, Restricted Stock Units Award Activity

The following table sets forth a summary of time-based restricted stock units and related information for the years ended December 31, 2017, 2016 and 2015:



	2017				2016				2015
	Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value
Unvested at beginning of year	257,096		$	57.47	175,411		$	74.52	185,457		$	73.87
Granted	100,993		71.33		162,256		44.45		81,022		69.05
Vested	(44,720	)	84.57		(60,015	)	72.41		(76,387	)	66.89
Forfeited	(23,315	)	57.52		(20,556	)	59.15		(14,681	)	75.73
Unvested at end of year	290,054		$	58.11	257,096		$	57.47	175,411		$	74.52

The following table sets forth a summary of performance-based awards for the year ended December 31, 2017:



	2017				2016				2015
	Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value		Awards		Weighted-Average Fair Value
Unvested at beginning of year	149,320		$	60.36	114,520		$	76.48	130,004		$	80.21
Granted	39,978		76.63		91,768		47.00		59,661		67.81
Vested	—		—		—		—		(38,869	)	72.25
Forfeited	(40,790	)	76.77		(56,968	)	71.25		(36,276	)	80.14
Unvested at end of year	148,508		$	60.23	149,320		$	60.36	114,520		$	76.48

Schedule of Share-based Payment Award, Performance-based Awards, Valuation Assumptions [Table Text Block]



	Year ended December 31,
	2017			2016			2015
Grant date share price	$	71.65		$	42.44		$	69.54
WESCO expected volatility	29		%	26		%	27		%
Peer group median volatility	24		%	24		%	23		%
Risk-free interest rate	1.5		%	0.9		%	1.1		%
Correlation	114		%	122		%	96		%

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Rental Payments for Operating Leases

Future minimum rental payments required under operating leases, primarily for real property that have noncancelable lease terms in excess of one year as of December 31, 2017, are as follows:



Years ending December 31	(In thousands)
2018	$	65,510
2019	55,378
2020	44,036
2021	33,498
2022	23,904
Thereafter	49,150

SEGMENTS AND RELATED INFORMATION (Tables)

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

Schedule of Revenue from External Customers and Long-Lived Assets, by Geographical Areas

WESCO attributes revenues from external customers to individual countries on the basis of the point of sale. The following table sets forth information about WESCO by geographic area:



	Net Sales Year Ended December 31,												Long-Lived Assets December 31,
	2017				2016				2015				2017		2016		2015
(In thousands)
United States	$	5,775,988	75	%	$	5,635,803	77	%	$	5,665,962	75	%	$	95,851	$	123,465	$	157,570
Canada	1,521,378		20	%	1,394,657		19	%	1,533,705		21	%	56,591		60,372		63,088
Mexico	77,280		1	%	62,430		1	%	70,048		1	%	262		227		332
Subtotal North American Operations	7,374,646				7,092,890				7,269,715				152,704		184,064		220,990
Other International	304,375		4	%	243,127		3	%	248,772		3	%	3,741		4,583		5,369
Total	$	7,679,021			$	7,336,017			$	7,518,487			$	156,445	$	188,647	$	226,359

Revenue from External Customers by Products and Services

The following table sets forth sales information about WESCO’s sales by product category:



Year Ended December 31,	2017	2016	2015
(percentages based on total sales)
General Supplies	40%	40%	40%
Wire, Cable and Conduit	15%	14%	15%
Communications and Security	15%	15%	15%
Electrical Distribution and Controls	10%	11%	11%
Lighting and Sustainability	12%	12%	10%
Automation, Controls and Motors	8%	8%	9%

OTHER FINANCIAL INFORMATION (Tables)

12 Months Ended

Dec. 31, 2017

Condensed Financial Information [Abstract]

Condensed Balance Sheet [Table Text Block]



	Condensed Consolidating Balance Sheet
	December 31, 2017
	(In thousands)
	WESCO International, Inc.		WESCO Distribution, Inc.		Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Cash and cash equivalents	$	—	$	50,602	$	67,351		$	—		$	117,953
Trade accounts receivable, net	—		—		1,170,080			—			1,170,080
Inventories	—		430,092		526,056			—			956,148
Prepaid expenses and other current assets	4,730		42,547		152,531			(35,140		)	164,668
Total current assets	4,730		523,241		1,916,018			(35,140		)	2,408,849
Intercompany receivables, net	—		—		2,189,136			(2,189,136		)	—
Property, buildings and equipment, net	—		50,198		106,247			—			156,445
Intangible assets, net	—		2,770		364,334			—			367,104
Goodwill	—		257,623		1,514,254			—			1,771,877
Investments in affiliates	3,058,613		5,023,826		—			(8,082,439		)	—
Other assets	—		2,778		28,415			—			31,193
Total assets	$	3,063,343	$	5,860,436	$	6,118,404		$	(10,306,715	)	$	4,735,468

Accounts payable	$	—	$	417,690	$	381,830		$	—		$	799,520
Short-term debt	—		—		34,075			—			34,075
Other current liabilities	—		80,039		162,475			(35,140		)	207,374
Total current liabilities	—		497,729		578,380			(35,140		)	1,040,969
Intercompany payables, net	939,784		1,249,352		—			(2,189,136		)	—
Long-term debt	—		934,033		379,228			—			1,313,261
Other noncurrent liabilities	3,820		120,709		140,566			—			265,095
Total WESCO International stockholders’ equity	2,119,739		3,058,613		5,023,826			(8,082,439		)	2,119,739
Noncontrolling interests	—		—		(3,596		)	—			(3,596		)
Total liabilities and stockholders’ equity	$	3,063,343	$	5,860,436	$	6,118,404		$	(10,306,715	)	$	4,735,468



	Condensed Consolidating Balance Sheet
	December 31, 2016
	(In thousands)
	WESCO International, Inc.		WESCO Distribution, Inc.		Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Cash and cash equivalents	$	—	$	41,552	$	68,579		$	—		$	110,131
Trade accounts receivable, net	—		—		1,034,402			—			1,034,402
Inventories	—		364,562		456,879			—			821,441
Prepaid expenses and other current assets	13,647		34,833		211,637			(123,013		)	137,104
Total current assets	13,647		440,947		1,771,497			(123,013		)	2,103,078
Intercompany receivables, net	—		—		2,060,336			(2,060,336		)	—
Property, buildings and equipment, net	—		51,824		105,783			—			157,607
Intangible assets, net	—		3,417		389,945			—			393,362
Goodwill	—		257,623		1,473,327			—			1,730,950
Investments in affiliates	3,538,476		4,028,502		—			(7,566,978		)	—
Other assets	—		23,846		22,998			—			46,844
Total assets	$	3,552,123	$	4,806,159	$	5,823,886		$	(9,750,327	)	$	4,431,841

Accounts payable	$	—	$	381,795	$	302,926		$	—		$	684,721
Short-term debt	—		—		20,920			—			20,920
Other current liabilities	—		120,299		170,872			(123,013		)	168,158
Total current liabilities	—		502,094		494,718			(123,013		)	873,799
Intercompany payables, net	1,572,486		487,850		—			(2,060,336		)	—
Long-term debt	—		983,449		379,686			—			1,363,135
Other noncurrent liabilities	12,737		55,898		162,641			—			231,276
Total WESCO International stockholders’ equity	1,966,900		2,776,868		4,790,110			(7,566,978		)	1,966,900
Noncontrolling interests	—		—		(3,269		)	—			(3,269		)
Total liabilities and stockholders’ equity	$	3,552,123	$	4,806,159	$	5,823,886		$	(9,750,327	)	$	4,431,841

Condensed Income Statement [Table Text Block]



	Condensed Consolidating Statement of Income and Comprehensive Income (Loss)
	Year ended December 31, 2015
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net sales	$	—		$	3,456,883		$	4,177,383		$	(115,779	)	$	7,518,487
Cost of goods sold (excluding depreciation and	—			2,784,413			3,356,192			(115,779		)	6,024,826
amortization)
Selling, general and administrative expenses	26			611,549			443,376			—			1,054,951
Depreciation and amortization	—			19,703			45,265			—			64,968
Results of affiliates’ operations	225,370			219,619			—			(444,989		)	—
Interest expense (income), net	24,910			63,261			(18,339		)	—			69,832
Provision for income taxes	(7,939		)	(6,929		)	110,405			—			95,537
Net income	$	208,373		$	204,505		$	240,484		$	(444,989	)	$	208,373
Less: Net loss attributable to noncontrolling interests	—			—			(2,314		)	—			(2,314		)
Net income attributable to WESCO International	$	208,373		$	204,505		$	242,798		$	(444,989	)	$	210,687
Other comprehensive income (loss):
Foreign currency translation adjustments	(225,795		)	(225,795		)	(225,795		)	451,590			(225,795		)
Post retirement benefit plan adjustments	4,532			4,532			4,532			(9,064		)	4,532
Comprehensive (loss) income attributable to WESCO International	$	(12,890	)	$	(16,758	)	$	21,535		$	(2,463	)	$	(10,576	)



	Condensed Consolidating Statement of Income and Comprehensive Income
	Year ended December 31, 2017
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net sales	$	—		$	3,370,088		$	4,441,655		$	(132,722	)	$	7,679,021
Cost of goods sold (excluding depreciation and	—			2,714,511			3,612,577			(132,722		)	6,194,366
amortization)
Selling, general and administrative expenses	—			555,503			544,245			—			1,099,748
Depreciation and amortization	—			18,442			45,575			—			64,017
Results of affiliates’ operations	160,587			168,782			—			(329,369		)	—
Interest expense (income), net	—			94,313			(25,863		)	—			68,450
Provision for income taxes	(2,546		)	(4,486		)	96,339			—			89,307
Net income	163,133			160,587			168,782			(329,369		)	163,133
Less: Net loss attributable to noncontrolling interests	—			—			(327		)	—			(327		)
Net income attributable to WESCO International	$	163,133		$	160,587		$	169,109		$	(329,369	)	$	163,460
Other comprehensive income (loss):
Foreign currency translation adjustments	85,762			85,762			85,762			(171,524		)	85,762
Post retirement benefit plan adjustments	(6,381		)	(6,381		)	(6,381		)	12,762			(6,381		)
Comprehensive income attributable to WESCO International	$	242,514		$	239,968		$	248,490		$	(488,131	)	$	242,841



	Condensed Consolidating Statement of Income and Comprehensive Income
	Year ended December 31, 2016
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net sales	$	—		$	3,306,265		$	4,134,508		$	(104,756	)	$	7,336,017
Cost of goods sold (excluding depreciation and	—			2,651,409			3,341,161			(104,756		)	5,887,814
amortization)
Selling, general and administrative expenses	61			477,437			571,788			—			1,049,286
Depreciation and amortization	—			20,226			46,632			—			66,858
Results of affiliates’ operations	240,571			155,814			—			(396,385		)	—
Interest expense (income), net	17,555			87,824			(28,804		)	—			76,575
Loss on debt redemption	123,933			—			—			—			123,933
Provision for income taxes	(2,098		)	8,263			24,266			—			30,431
Net income	101,120			216,920			179,465			(396,385		)	101,120
Less: Net loss attributable to noncontrolling interests	—			—			(468		)	—			(468		)
Net income attributable to WESCO International	$	101,120		$	216,920		$	179,933		$	(396,385	)	$	101,588
Other comprehensive income (loss):
Foreign currency translation adjustments	38,275			38,275			38,275			(76,550		)	38,275
Post retirement benefit plan adjustments	(2,485		)	(2,485		)	(2,485		)	4,970			(2,485		)
Comprehensive income attributable to WESCO International	$	136,910		$	252,710		$	215,723		$	(467,965	)	$	137,378

Condensed Cash Flow Statement [Table Text Block]



	Condensed Consolidating Statement of Cash Flows
	Year ended December 31, 2016
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net cash provided by (used in) operating activities	$	95,388		$	(243,476	)	$	448,323		$	—		$	300,235
Investing activities:
Capital expenditures	—			(12,482		)	(5,475		)	—			(17,957		)
Acquisition payments, net of cash acquired	—			(50,890		)	—			—			(50,890		)
Proceeds from sale of assets	—			—			8,361			—			8,361
Dividends received from subsidiaries	—			82,912			—			(82,912		)	—
Advances to subsidiaries and other	—			(297,259		)	(337,344		)	624,603			(10,000		)
Net cash used in investing activities	—			(277,719		)	(334,458		)	541,691			(70,486		)
Financing activities:
Proceeds from issuance of debt	252,246			1,566,864			672,345			(297,259		)	2,194,196
Repayments of debt	(344,804		)	(1,030,520		)	(752,401		)	(327,344		)	(2,455,069		)
Equity activities	(2,830		)	—			—			—			(2,830		)
Dividends paid by subsidiaries	—			—			(82,912		)	82,912			—
Other	—			(12,560		)	—			—			(12,560		)
Net cash (used in) provided by financing activities	(95,388		)	523,784			(162,968		)	(541,691		)	(276,263		)
Effect of exchange rate changes on cash and cash equivalents	—			—			(3,634		)	—			(3,634		)
Net change in cash and cash equivalents	—			2,589			(52,737		)	—			(50,148		)
Cash and cash equivalents at the beginning of period	—			38,963			121,316			—			160,279
Cash and cash equivalents at the end of period	$	—		$	41,552		$	68,579		$	—		$	110,131



	Condensed Consolidating Statement of Cash Flows
	Year ended December 31, 2017
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net cash (used in) provided by operating activities	$	(36,575	)	$	101,826		$	83,871		$	—		$	149,122
Investing activities:
Capital expenditures	—			(13,215		)	(8,292		)	—			(21,507		)
Proceeds from sale of assets	—			—			6,766			—			6,766
Dividends received from subsidiaries	—			307,784			—			(307,784		)	—
Advances to subsidiaries and other	—			(383,686		)	26,912			366,220			9,446
Net cash (used in) provided by investing activities	—			(89,117		)	25,386			58,436			(5,295		)
Financing activities:
Proceeds from issuance of debt	143,367			775,926			1,144,848			(383,686		)	1,680,455
Repayments of debt	—			(785,392		)	(952,740		)	17,466			(1,720,666		)
Equity activities	(106,792		)	—			—			—			(106,792		)
Dividends paid by subsidiaries	—			—			(307,784		)	307,784			—
Other	—			5,807			—			—			5,807
Net cash provided by (used in) financing activities	36,575			(3,659		)	(115,676		)	(58,436		)	(141,196		)
Effect of exchange rate changes on cash and cash equivalents	—			—			5,191			—			5,191
Net change in cash and cash equivalents	—			9,050			(1,228		)	—			7,822
Cash and cash equivalents at the beginning of period	—			41,552			68,579			—			110,131
Cash and cash equivalents at the end of period	$	—		$	50,602		$	67,351		$	—		$	117,953



	Condensed Consolidating Statement of Cash Flows
	Year ended December 31, 2015
	(In thousands)
	WESCO International, Inc.			WESCO Distribution, Inc.			Non-Guarantor Subsidiaries			Consolidating and Eliminating Entries			Consolidated
Net cash provided by operating activities	$	3,531		$	214,037		$	65,481		$	—		$	283,049
Investing activities:
Capital expenditures	—			(15,266		)	(6,392		)	—			(21,658		)
Acquisition payments, net of cash acquired	—			(151,595		)	—			—			(151,595		)
Proceeds from sale of assets	—			—			3,023			—			3,023
Dividends received from subsidiaries	—			114,101			—			(114,101		)	—
Advances to subsidiaries and other	—			(197,345		)	17,461			179,884			—
Net cash (used in) provided by investing activities	—			(250,105		)	14,092			65,783			(170,230		)
Financing activities:
Proceeds from issuance of debt	150,705			1,224,596			452,655			(197,345		)	1,630,611
Repayments of debt	—			(1,175,056		)	(379,578		)	17,461			(1,537,173		)
Equity activities	(154,236		)	—			—			—			(154,236		)
Dividends paid by subsidiaries	—			—			(114,101		)	114,101			—
Other	—			(7,017		)	—			—			(7,017		)
Net cash provided by (used in) financing activities	(3,531		)	42,523			(41,024		)	(65,783		)	(67,815		)
Effect of exchange rate changes on cash and cash equivalents	—			—			(13,044		)	—			(13,044		)
Net change in cash and cash equivalents	—			6,455			25,505			—			31,960
Cash and cash equivalents at the beginning of period	—			32,508			95,811			—			128,319
Cash and cash equivalents at the end of period	$	—		$	38,963		$	121,316		$	—		$	160,279

SELECTED QUARTERLY FINANCIAL DATA (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information



	First Quarter		Second Quarter		Third Quarter			Fourth Quarter
2017
Net Sales	$	1,772,591	$	1,909,624	$	2,000,159		$	1,996,647
Cost of goods sold (excluding depreciation and amortization)	1,422,573		1,543,510		1,614,814			1,613,469
Income from operations	67,089		83,105		89,250			81,446
Income before income taxes	50,368		66,289		71,939			63,844
Net income⁽¹⁾	37,800		49,535		53,576			22,222
Net income attributable to WESCO International⁽¹⁾	37,729		49,510		53,675			22,546
Basic earnings per share attributable to WESCO International ^{(1) (2)}	0.77		1.03		1.13			0.48
Diluted earnings per share attributable to WESCO International ^{(1) (3)}	0.76		1.02		1.12			0.47

2016
Net Sales	$	1,775,961	$	1,911,582	$	1,855,212		$	1,793,262
Cost of goods sold (excluding depreciation and amortization)	1,420,793		1,532,113		1,490,173			1,444,735
Income from operations	69,508		87,987		92,555			82,009
Income (loss) before income taxes	50,679		68,535		(52,170		)	64,507
Net income (loss)	34,534		49,852		(31,021		)	47,755
Net income (loss) attributable to WESCO International	36,053		49,798		(31,611		)	47,348
Basic earnings (loss) per share attributable to WESCO International ^{(2) (4)}	0.85		1.18		(0.73		)	0.97
Diluted earnings (loss) per share attributable to WESCO International ^{(3) (4)}	0.77		1.02		(0.73		)	0.96

Schedule II - Valuation and Qualifying Accounts (Tables)

12 Months Ended

Dec. 31, 2016

Valuation and Qualifying Accounts [Abstract]

Allowance for Doubtful Accounts



	Balance at Beginning		Charged to	Charged to Other			Balance at
	of Period		Expense	Accounts⁽¹⁾	Deductions⁽²⁾		End of Period
Allowance for doubtful accounts	(In thousands)
Year ended December 31, 2017	$	22,007	8,466	—	(9,160	)	$	21,313
Year ended December 31, 2016	22,587		5,888	21	(6,489	)	22,007
Year ended December 31, 2015	21,084		6,099	1,305	(5,901	)	22,587

_________________________


⁽¹⁾	Represents allowance for doubtful accounts in connection with certain acquisitions and divestitures.


⁽²⁾	Includes a reduction in the allowance for doubtful accounts due to write-off of accounts receivable.

ORGANIZATION (Details)	Dec. 31, 2017 customers countries branches
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Customers (in customers) \| customers	70,000
Full service branches (in branches) \| branches	500
Additional countries (in countries) \| countries	15

ACCOUNTING POLICIES - GENERAL (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 27, 2009
Defined Benefit Plan Disclosure
Debt Instrument, Face Amount	$ 1,360,000	$ 1,400,000
Income Taxes Receivable, Current	23,250	5,725
Accrued Income Taxes, Current	9,712	9,881
Prepaid Expense and Other Assets, Current	40,189	46,360
Assets, Current	2,408,849	2,103,078
Intangible Assets, Net (Including Goodwill)	$ 1,872,100	1,827,900
Characteristics of Securitizations or Asset-backed Financing Arrangements that are Accounted for as Sale	WESCO maintains control of the receivables transferred pursuant to its accounts receivable securitization program (the “Receivables Facility”); therefore, the transfers do not qualify for “sale” treatment. As a result, the transferred receivables remain on the balance sheet, and WESCO recognizes the related secured borrowing. The expenses associated with the Receivables Facility are reported as interest expense in the Consolidated Statements of Income and Comprehensive Income (Loss).
Self Insurance Reserve	$ 13,900	9,500
Sales Revenue, Services, Net	26,200	27,100	$ 35,100
Shipping, Handling and Transportation Costs	61,800	57,900	59,400
Allowance for Doubtful Accounts Receivable, Current	21,313	22,007
Provision for Doubtful Accounts	8,500	5,900	6,100
Inventory Valuation Reserves	28,600	27,300
Inventory Write-down	8,800	7,300	8,600
General and Administrative Costs in Inventory, Amount Incurred	70,700	65,300
Increase in deferred charges	915	6,002	3,359
Deferred Finance Costs, Net	13,711	16,508
Stock-based compensation expense	14,809	12,493	12,899
Debt Instrument, Fair Value Disclosure	1,386,400	1,416,000
Goodwill	1,771,877	1,730,950	$ 1,681,662
Assets	4,735,468	4,431,841
Liabilities, Current	1,040,969	873,799
Deferred Tax Liabilities, Net, Noncurrent	136,858	168,245
Liabilities	2,619,325	2,468,210
Retained Earnings (Accumulated Deficit)	2,079,697	1,914,757		$ 1,602,139
Accumulated Other Comprehensive Income (Loss), Net of Tax	(312,590)	(391,971)		$ (206,498)
Stockholders' Equity Attributable to Parent	2,119,739	1,966,900
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,116,143	1,963,631
Impact of Restatement on Opening Retained Earnings, Net of Tax		(46,381)
Liabilities and Equity	$ 4,735,468	$ 4,431,841
Pension Plan [Member]
Defined Benefit Plan Disclosure
Discount rate	3.50%	3.90%
Minimum
Defined Benefit Plan Disclosure
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	2 years
Maximum
Defined Benefit Plan Disclosure
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	20 years
Convertible Debt
Defined Benefit Plan Disclosure
Debt Instrument, Face Amount					$ 345,000
Scenario, Previously Reported [Member]
Defined Benefit Plan Disclosure
Income Taxes Receivable, Current		$ 72,881
Accrued Income Taxes, Current		32,879
Prepaid Expense and Other Assets, Current		48,583
Assets, Current		2,172,457
Goodwill		1,720,714
Assets		4,490,984
Liabilities, Current		896,797
Deferred Tax Liabilities, Net, Noncurrent		158,009
Liabilities		2,480,972
Retained Earnings (Accumulated Deficit)		1,956,532
Accumulated Other Comprehensive Income (Loss), Net of Tax		(387,365)
Stockholders' Equity Attributable to Parent		2,013,281
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		2,010,012
Liabilities and Equity		4,490,984
Scenario, Adjustment [Member]
Defined Benefit Plan Disclosure
Income Taxes Receivable, Current		(67,156)
Accrued Income Taxes, Current		(22,998)
Prepaid Expense and Other Assets, Current		(2,223)
Assets, Current		(69,379)
Goodwill		10,236
Assets		(59,143)
Liabilities, Current		(22,998)
Deferred Tax Liabilities, Net, Noncurrent		10,236
Liabilities		(12,762)
Retained Earnings (Accumulated Deficit)		(41,775)
Accumulated Other Comprehensive Income (Loss), Net of Tax		(4,606)
Stockholders' Equity Attributable to Parent		(46,381)
Liabilities and Equity		(59,143)
Restatement Adjustment [Member]
Defined Benefit Plan Disclosure
Income Taxes Receivable, Current		5,725
Accrued Income Taxes, Current		9,881
Prepaid Expense and Other Assets, Current		46,360
Assets, Current		2,103,078
Goodwill		1,730,950
Assets		4,431,841
Liabilities, Current		873,799
Deferred Tax Liabilities, Net, Noncurrent		168,245
Liabilities		2,468,210
Retained Earnings (Accumulated Deficit)		1,914,757
Accumulated Other Comprehensive Income (Loss), Net of Tax		(391,971)
Stockholders' Equity Attributable to Parent		1,966,900
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		1,963,631
Liabilities and Equity		$ 4,431,841

ACCOUNTING POLICIES - PPE (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Intangible Assets, Net (Including Goodwill)	$ 1,872,100	$ 1,827,900
Property, buildings and equipment, net	$ 156,445	$ 157,607
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	five to forty years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	three to ten years
Software and Software Development Costs [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Estimated Useful Lives	three to five years
Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Property, buildings and equipment, net	$ 97,600

GOODWILL AND INTANGIBLE ASSETS - TEXTUALS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill, Purchase Accounting Adjustments	$ 0	$ 17,618
Finite-Lived Intangible Assets, Amortization Expense	$ 37,800	$ 39,100	$ 36,900

GOODWILL AND INTANGIBLE ASSETS - SCHEDULE OF GOODWILL (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning balance January 1	$ 1,730,950	$ 1,681,662
Goodwill, Translation Adjustments	40,927	21,434
Ending balance December 31	1,771,877	1,730,950
Goodwill, Purchase Accounting Adjustments	0	17,618
Scenario, Adjustment [Member]
Goodwill [Roll Forward]
Beginning balance January 1	10,236
Ending balance December 31		10,236
Goodwill, Period Increase (Decrease)	$ 0	$ 10,236

GOODWILL AND INTANGIBLE ASSETS - SCHEDULE OF INTANGIBLE ASSETS BY MAJOR CLASS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	$ (223,554)	$ (178,813)
Intangible Assets Gross Excluding Goodwill	590,658	572,175
Intangible Assets, Net (Excluding Goodwill)	367,104	393,362
Trademarks
Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	25,118	25,098
Finite-Lived Intangible Assets, Accumulated Amortization	(5,516)	(3,426)
Finite-Lived Intangible Assets, Net	19,602	21,672
Non-compete agreements
Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	196	196
Finite-Lived Intangible Assets, Accumulated Amortization	(102)	(63)
Finite-Lived Intangible Assets, Net	94	133
Customer relationships
Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	377,270	362,637
Finite-Lived Intangible Assets, Accumulated Amortization	(161,711)	(126,835)
Finite-Lived Intangible Assets, Net	215,559	235,802
Distribution agreements
Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	39,515	38,972
Finite-Lived Intangible Assets, Accumulated Amortization	(22,200)	(19,295)
Finite-Lived Intangible Assets, Net	17,315	19,677
Patents
Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	48,310	48,310
Finite-Lived Intangible Assets, Accumulated Amortization	(34,025)	(29,194)
Finite-Lived Intangible Assets, Net	$ 14,285	19,116
Finite-Lived Intangible Assets, Useful Life, Minimum	10 years
Trademarks
Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets, Gross	$ 100,249	$ 96,962
Minimum
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	2 years
Minimum \| Trademarks
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	4 years	12 years
Minimum \| Non-compete agreements
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	2 years
Minimum \| Customer relationships
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	2 years	11 years
Minimum \| Distribution agreements
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years
Maximum
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	20 years
Maximum \| Trademarks
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	15 years	13 years
Maximum \| Non-compete agreements
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years
Maximum \| Customer relationships
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	20 years	14 years
Maximum \| Distribution agreements
Intangible Assets [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	19 years

GOODWILL AND INTANGIBLE ASSETS - SCHEDULE OF EXPECTED AMORTIZATION EXPENSE (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Finite-Lived Intangible Assets, Future Amortization Expense [Abstract]
2014	$ 37,848
2015	36,549
2016	34,621
2017	26,930
2018	$ 24,350

CONCENTRATIONS OF CREDIT RISK AND SIGNIFICANT SUPPLIERS (Details)	12 Months Ended
	Dec. 31, 2017
Supplier Concentration Risk \| Cost of Goods, Total
Concentration Risk [Line Items]
Concentration Risk, Percentage	0.00%

ACQUISITIONS - ACQUISITIONS (Details) $ in Thousands	3 Months Ended								12 Months Ended
ACQUISITIONS - ACQUISITIONS (Details) $ in Thousands	Dec. 31, 2017 USD ($) branches	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) branches	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) branches	Dec. 31, 2016 USD ($) branches	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Number of Stores \| branches	500								500
Net loss attributable to noncontrolling interest									$ (327)	$ (468)	$ (2,314)
Net sales	$ 1,996,647	$ 2,000,159	$ 1,909,624	$ 1,772,591	$ 1,793,262	$ 1,855,212	$ 1,911,582	$ 1,775,961	7,679,021	7,336,017	7,518,487
Payments to Acquire Businesses, Gross									$ 0	$ 51,922	$ 152,263
Atlanta Electrical Distributors, LLC (AED) [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition									Mar. 14, 2016
Business Acquisition, Name of Acquired Entity									Atlanta Electrical Distributors, LLC
Number of Stores	5								5
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period									$ 85,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	$ 21,800								21,800
Goodwill, Acquired During Period									$ 30,000
Needham Electric Supply Corporation [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition										Oct. 30, 2015
Business Acquisition, Name of Acquired Entity										Needham Electric Supply Corporation ("Needham")
Number of Stores					24					24
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period										$ 115,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill					$ 31,000					31,000
Goodwill, Acquired During Period										$ 35,700
Hill Country Electric Supply [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition										May 01, 2015
Business Acquisition, Name of Acquired Entity										Hill Country Electric Supply, LP ("Hill Country")
Number of Stores \| branches					9					9
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period										$ 140,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill					$ 21,100					21,100
Goodwill, Acquired During Period										16,200
Other Intangible Assets [Member]
Business Acquisition [Line Items]
Finite-lived Intangible Assets Acquired										200
Trademarks
Business Acquisition [Line Items]
Finite-lived Intangible Assets Acquired										14,300
Trademarks \| Atlanta Electrical Distributors, LLC (AED) [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									13 years
Finite-lived Intangible Assets Acquired									$ 6,000
Customer Relationships
Business Acquisition [Line Items]
Finite-lived Intangible Assets Acquired										$ 37,600
Customer Relationships \| Atlanta Electrical Distributors, LLC (AED) [Member]
Business Acquisition [Line Items]
Finite-lived Intangible Assets Acquired									$ 15,800
Noncompete Agreements [Member] \| Atlanta Electrical Distributors, LLC (AED) [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									5 years
Finite-lived Intangible Assets Acquired									$ 100
Maximum [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									20 years
Maximum [Member] \| Trademarks
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									15 years	13 years
Maximum [Member] \| Customer Relationships
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									20 years	14 years
Maximum [Member] \| Customer Relationships \| Atlanta Electrical Distributors, LLC (AED) [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									14 years
Maximum [Member] \| Noncompete Agreements [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									7 years
Minimum
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									2 years
Minimum \| Trademarks
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									4 years	12 years
Minimum \| Customer Relationships
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									2 years	11 years
Minimum \| Customer Relationships \| Atlanta Electrical Distributors, LLC (AED) [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									13 years
Minimum \| Noncompete Agreements [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									2 years

ACQUISITIONS - SCHEDULE OF BUSINESS ACQUISTIONS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 0	$ 76,980	$ 192,099
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0	25,058	39,836
Cash paid for acquisitions	0	51,922	152,263
Cash Acquired from Acquisition	0	(1,032)	(668)
Supplemental cash flow disclosure related to acquisitions:
Payments to Acquire Businesses, Net of Cash Acquired	$ 0	$ 50,890	$ 151,595

ACQUISITIONS - SCHEDULE OF PURCHASE PRICE ALLOCATION (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets Acquired
Goodwill	$ 1,771,877	$ 1,730,950	$ 1,681,662
Total assets acquired	0	76,980	192,099
Liabilities Assumed
Total liabilities assumed	$ 0	$ 25,058	$ 39,836

PROPERTY, BUILDINGS AND EQUIPMENT - SCHEDULE OF PROPERTY, BUILDINGS AND EQUIPMENT (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Accumulated depreciation and amortization	$ (278,455)	$ (259,126)
Property, buildings and equipment, net	156,445	157,607
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, buildings and equipment, gross	117,894	117,461
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, buildings and equipment, gross	183,801	178,183
Software and Software Development Costs [Member]
Property, Plant and Equipment [Line Items]
Property, buildings and equipment, gross	103,842	93,040
Depreciable [Member]
Property, Plant and Equipment [Line Items]
Property, buildings and equipment, gross	405,537	388,684
Property, buildings and equipment, net	127,082	129,558
Land
Property, Plant and Equipment [Line Items]
Property, buildings and equipment, gross	25,814	24,653
Construction in progress
Property, Plant and Equipment [Line Items]
Property, buildings and equipment, gross	$ 3,549	$ 3,396

PROPERTY, BUILDINGS AND EQUIPMENT - PROPERTY, BUILDINGS AND EQUIPMENT (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 16.3	$ 17.1	$ 17.8
Capitalized software amortization	9.9	10.6	$ 10.3
Unamortized software cost	22.4	21.6
Capitalized leases	10.6	12.0
Accumulated amortization related to capitalized leases	$ 9.0	$ 8.9

DEBT - SCHEDULE OF DEBT (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Aug. 27, 2009
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,360,000	$ 1,400,000
Long-term debt	1,362,271
Total debt	1,362,271	1,401,781
Debt Issuance Costs, Net	(13,711)	(16,508)
Less current and short-term portion	35,299	22,138
Total long-term debt	1,313,261	1,363,135
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term debt	84,237	143,980
Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	500,000
Long-term debt	500,000	500,000
Senior Notes due 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	350,000
Long-term debt	$ 350,000	350,000
Accounts receivable securitization facility
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	1.90%
Long-term debt	$ 380,000	380,000
Revolving credit facility
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	3.00%
Long-term debt	$ 12,000	4,000
Foreign Line of Credit [Member]
Debt Instrument [Line Items]
Long-term debt	$ 34,075	20,920
Convertible Debt
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 345,000
Debt Instrument, Interest Rate, Effective Percentage	13.875%
Capital leases
Debt Instrument [Line Items]
Long-term debt	$ 1,959	$ 2,881
United States \| Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	4.70%
Debt Instrument, Face Amount	$ 700,000
Debt Instrument, Interest Rate, Effective Percentage	1.00%

DEBT - DEBT (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		8 Months Ended	9 Months Ended	12 Months Ended								24 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Aug. 27, 2009	Sep. 30, 2017	Jun. 14, 2022	Jun. 14, 2021	Jun. 14, 2020	Dec. 14, 2019	Dec. 14, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 15, 2024	Dec. 15, 2021
Debt Instrument [Line Items]
Accounts Receivable from Securitization										$ 380,000
Long-term debt										1,362,271
Letters of Credit Outstanding, Amount										18,000	$ 20,100
Gain (Loss) on Extinguishment of Debt	$ (124,000)			$ 123,900						0	(123,933)	$ 0
Debt Instrument, Face Amount										1,360,000	1,400,000
Debt discount on convertible debentures										(513)
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt			$ 106,500								(139,765)	(2)
Debt Instrument, Convertible, Carrying Amount of Equity Component										344,800
Amortization of Financing Costs and Discounts										$ 3,984	$ 6,684	12,195
Debt Instrument, Call Date, Earliest										Sep. 15, 2016
2014										$ 37,848
2015										36,549
2016										34,621
2017										26,930
2018										$ 24,350
Debt Instrument, Convertible, Terms of Conversion Feature										Holders who surrendered the 2029 Debentures for conversion received 18 shares of WESCO stock for each $1,000 principal amount of 2029 Debentures converted.
Debt Conversion, Converted Instrument, Shares Issued										6,267,688
Foreign Line of Credit [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate										5.42%	7.36%
Long-term debt										$ 34,075	$ 20,920
Term Loan Facility
Debt Instrument [Line Items]
Debt discount on convertible debentures											(1,026)
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term debt										$ 500,000	500,000
Debt Instrument, Interest Rate, Stated Percentage										5.375%
Deferred Finance Costs, Gross											8,400
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed										101.00%
Debt Instrument, Face Amount										$ 500,000
Senior Notes due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt										$ 350,000	350,000
Debt Instrument, Interest Rate, Stated Percentage										5.375%
Deferred Finance Costs, Gross											6,000
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed										101.00%
Debt Instrument, Face Amount										$ 350,000
Secured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date										Dec. 12, 2012
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum										On November 19, 2013, the Borrowers and WESCO International entered into an amendment (the “Term Loan Amendment”) to the Term Loan Agreement. The Term Loan Amendment, among other things, reduced the applicable margin on U.S. term loans by 0.50% and the LIBOR floor applicable to the U.S. sub-facility from 1.00% to 0.75%. The modified pricing terms were effective December 13, 2013.
Debt Instrument, Unused Borrowing Capacity, Amount										$ 300,000
Debt Instrument, Collateral										The Borrowers' obligations under the Term Loan Facility are secured by substantially all of the assets of the Borrowers, the Company and certain of the Company's other subsidiaries; provided that, with respect to borrowings under the U.S. sub-facility, the collateral does not include assets of certain foreign subsidiaries or more than 65% of the issued and outstanding equity interests in certain foreign subsidiaries.
Debt Instrument, Decrease, Repayments		$ 500,000
Long-term debt										$ 84,237	143,980
Accounts Receivable Securitization Facility [Member]
Debt Instrument [Line Items]
Amendment Description										On November 8, 2017, WESCO Distribution amended its Receivables Facility pursuant to the terms and conditions of a Fifth Amendment to Fourth Amended and Restated Receivables Purchase Agreement, by and among WESCO Receivables Corp. (“WESCO Receivables”), WESCO Distribution, the various purchasers from time to time party thereto and PNC Bank, National Association, as Administrator (the "Amendment"). The Amendment extended the term of the Receivables Facility to September 24, 2020 and added and amended certain other defined terms. Substantially all other terms and conditions of the Receivables Facility were unchanged.
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate										1.90%
Accounts Receivable Eligible for Securitization										$ 751,200	657,500
Long-term debt										$ 380,000	380,000
Revolving credit facility
Debt Instrument [Line Items]
Line of Credit Facility, Borrowing Capacity, Description										On September 24, 2015, WESCO International, WESCO Distribution and certain other subsidiaries of the Company entered into a $600 million revolving credit facility (the “Revolving Credit Facility”), which contains a letter of credit sub-facility of up to $125 million, pursuant to the terms and conditions of a Second Amended and Restated Credit Agreement (the "Credit Agreement"). The Revolving Credit Facility contains an accordion feature allowing WESCO Distribution to request increases to the borrowing commitments of up to $200 million in the aggregate, subject to customary conditions.
Debt Instrument, Maturity Date										Sep. 01, 2020
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate										3.00%
Debt Instrument, Increase, Additional Borrowings										$ 834,400	1,025,800
Debt Instrument, Decrease, Repayments										826,400	1,096,800
Long-term debt										12,000	4,000
Line of Credit Facility, Remaining Borrowing Capacity										$ 562,900	509,700
Convertible Debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate at Period End										13.875%
Debt Instrument, Face Amount			345,000
Debt discount on convertible debentures			$ (181,200)							$ 0	(163,316)
Amortization of Financing Costs and Discounts											$ 3,100	$ 4,200
Canada \| Secured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate at Period End										2.00%
Debt Instrument, Face Amount										$ 150,000
Canada \| Revolving credit facility
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity										$ 400,000
United States \| Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate										4.70%
Debt Instrument, Interest Rate at Period End										1.00%
Long-term Debt, Gross										$ 84,800
Debt Instrument, Face Amount										700,000
United States \| Revolving credit facility
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity										600,000
Minimum \| Foreign Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity										300
Maximum \| Foreign Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity										$ 21,000
Common Stock
Debt Instrument [Line Items]
Common Stock, Par or Stated Value Per Share										$ 0.01	$ 0.01
London Interbank Offered Rate (LIBOR) [Member] \| Minimum \| Revolving credit facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate										1.25%
London Interbank Offered Rate (LIBOR) [Member] \| Maximum \| Revolving credit facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate										1.75%
Prime Rate [Member] \| Minimum \| Revolving credit facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate										0.25%
Prime Rate [Member] \| Maximum \| Revolving credit facility
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate										0.75%
Scenario, Forecast [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed								101.344%	102.688%					100.00%
Scenario, Forecast [Member] \| Senior Notes due 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed					101.344%	102.688%	104.031%						100.00%

DEBT - OUTSTANDING CONVERTIBLE DEBENTURE SCHEDULE (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Aug. 27, 2009
Debt Instrument [Line Items]
Principal Balance	$ 1,362,784
Debt Instrument, Unamortized Discount	(513)
Total long-term debt	1,362,271
Convertible Debt
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount	$ 0	$ (163,316)	$ (181,200)

DEBT - SCHEDULE OF MATURITIES OF LONG-TERM DEBT (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract]
2013	$ 35,299
2014	85,246
2015	392,239
2016	500,000
2017	0
Thereafter	350,000
Total payments on debt	1,362,784
Debt discount on convertible debentures	(513)
Total long-term debt	$ 1,362,271

CAPITAL STOCK (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Preferred Stock, Shares Authorized	20,000,000	20,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 344.8
Common Stock
Class of Stock [Line Items]
Common Stock, Shares Authorized	210,000,000	210,000,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common Class B
Class of Stock [Line Items]
Common Stock, Shares Authorized	20,000,000	20,000,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01

INCOME TAXES - ADDITIONAL INFORMATION (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	$ 56,400
Income Tax Examination, Description	The statutes of limitation with respect to the Company’s 2004 to 2007 U.S. federal income tax returns are open by waiver only in connection with the implementation of the Mutual Agreement Procedure (“MAP”) concluded in the fourth quarter of 2015 between the Competent Authorities of the Internal Revenue Service (the "IRS") and the Canada Revenue Agency (the “CRA”) and the request for MAP assistance filed with the IRS and CRA in 2017 with respect to certain other 2004 to 2007 transfer pricing matters. The statutes of limitation with respect to the Company’s 2008 to 2011 U.S. federal income tax returns are open by waiver only in connection with the implementation of the Advance Pricing Agreement (“APA”) concluded in the fourth quarter of 2015 between the IRS and CRA and certain other transfer pricing matters pending with the CRA. The statutes of limitation with respect to the Company’s 2012 and 2013 U.S. federal income tax returns are open by waiver only in connection with the IRS examination of the 2012 and 2013 years and the APA.
Adjustments to Additional Paid in Capital, Income Tax Deficiency from Share-based Compensation		$ (100)
Deferred Tax Assets, Operating Loss Carryforwards	$ 19,128	18,440
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation			$ 1,600
Undistributed Earnings of Foreign Subsidiaries	884,000
Unrecognized Tax Benefits	4,348	6,181	5,436	$ 20,033
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	1,700	7,500	$ 6,200
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	3,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	$ 1,800	$ 2,200
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount	$ 82,800
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	0	$ 15,698
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Tax Credit Carryforwards, Foreign		15,700
Taxes Payable	65,000
Canada Revenue Agency [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards, Foreign	10,400	10,000
Unrecognized Tax Benefits		6,181	$ 5,436
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards, Foreign	7,000	6,200
Deferred Tax Assets, Tax Credit Carryforwards	17,800
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	3,100	$ 3,200
Canada
Operating Loss Carryforwards [Line Items]
Undistributed Earnings of Foreign Subsidiaries	$ 807,200
Minimum \| Canada Revenue Agency [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2029
Minimum \| Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2019
Minimum \| State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2022
Settlement with Taxing Authority [Member]
Operating Loss Carryforwards [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	$ 300

INCOME TAXES - SCHEDULE OF COMPONENTS OF INCOME TAX EXPENSE (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current taxes
Federal	$ 122,170	$ 65,614	$ 45,812
State	2,259	6,489	4,565
Foreign	15,274	3,502	2,309
Total current	139,703	75,605	52,686
Deferred taxes
Federal	(48,060)	(42,835)	29,593
State	4,508	(2,938)	3,767
Foreign	(6,844)	599	9,491
Total deferred	(50,396)	(45,174)	42,851
Income tax expense	$ 89,307	$ 30,431	$ 95,537

INCOME TAXES - SCHEDULE OF INCOME BEFORE INCOME TAX, DOMESTIC AND FOREIGN (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States									$ 180,957	$ 80,881	$ 288,881
Foreign									71,483	50,670	15,029
Income before income taxes	$ 63,844	$ 71,939	$ 66,289	$ 50,368	$ 64,507	$ (52,170)	$ 68,535	$ 50,679	$ 252,440	$ 131,551	$ 303,910

INCOME TAXES - SCHEDULE OF EFFECTIVE INCOME TAX RATE RECONCILIATION (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
State taxes, net of federal tax benefit	1.40%	1.00%	2.20%
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Percent	32.80%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	(22.40%)	0.00%	0.00%
Nondeductible expenses	(0.10%)	(1.60%)	(1.20%)
Foreign tax rate differences	(1.30%)	(0.40%)	(1.10%)
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent	(10.50%)	(19.90%)	(8.80%)
Effective Income Tax Rate Reconciliation, Tax Contingency, Percent	0.00%	3.70%	2.70%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent	0.40%	1.10%	0.00%
Other	0.10%	1.00%	0.20%
Effective income tax rate	35.40%	23.10%	31.40%

INCOME TAXES - SCHEDULE OF DEFERRED TAX ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax Credit Carryforward [Line Items]
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Percent	32.80%	0.00%	0.00%
Deferred Income Tax Expense (Benefit)	$ (50,396)	$ (45,174)	$ 42,850
Deferred Tax Assets, Net [Abstract]
Accounts receivable	3,496	3,484
Employee benefits	14,835	18,577
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost	16,341	23,844
Deferred Tax Assets, Tax Deferred Expense, Other	8,456	22,056
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	0	15,698
Tax loss carryforwards	19,128	18,440
Other	11,850	7,175
Deferred taxes, Assets	71,588	107,844
Deferred Tax Liabilities, Inventory	3,181	4,001
Deferred Tax Liabilities [Abstract]
Depreciation	13,283	11,487
Amortization of intangible assets	159,107	237,015
Other	8,672	7,783
Deferred Tax Assets, Gross	74,106	109,274
Deferred Tax Liabilities, Gross	184,243	260,286
Deferred Tax Assets, Valuation Allowance	(2,518)	(1,430)
Foreign Tax Authority
Deferred Tax Liabilities [Abstract]
Deferred Tax Assets, Tax Credit Carryforwards	$ 17,800
Foreign Tax Authority \| Minimum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2019
State and Local Jurisdiction [Member]
Deferred Tax Assets, Net [Abstract]
Tax loss carryforwards	$ 3,100	3,200
State and Local Jurisdiction [Member] \| Minimum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2022
State and Local Jurisdiction [Member] \| Maximum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2036
Internal Revenue Service (IRS) [Member]
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets, Tax Credit Carryforwards, Foreign		$ 15,700

INCOME TAXES - NOLS (Details) - USD ($) $ in Thousands	12 Months Ended
INCOME TAXES - NOLS (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	$ 19,128	$ 18,440
Canada Revenue Agency [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	10,400	10,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	3,100	3,200
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	7,000	$ 6,200
Operating Loss Carryforwards, Valuation Allowance	$ 2,500
Minimum \| Canada Revenue Agency [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2029
Minimum \| State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2022
Minimum \| Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2019
Maximum \| Canada Revenue Agency [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2037
Maximum \| State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2036

INCOME TAXES - SCHEDULE OF INCOME TAX CONTINGENCIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Deferred Tax Assets, Tax Deferred Expense, Other	$ 8,456	$ 22,056
Unrecognized Tax Benefits, Interest on Income Taxes Expense	(100)	(1,200)	$ (8,700)
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance January 1	6,181	5,436	20,033
Additions based on tax positions related to the current year	0	0	46
Additions for tax positions of prior years	0	3,298	402
Reductions for tax positions of prior years	(155)	(21)	(378)
Settlements	(1,025)	(1,921)	(9,638)
Lapse in statute of limitations	(755)	(728)	(1,497)
Ending balance December 31	4,348	6,181	5,436
Unrecognized Tax Benefits, Decrease Resulting from Foreign Currency Translation	$ 102	$ 117	$ 3,532

EARNINGS PER SHARE - SCHEDULE OF EARNINGS PER SHARE, BASIC AND DILUTED (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accelerated Share Repurchases [Line Items]
Payments for Repurchase of Common Stock									$ 106,792	$ 4,818	$ 155,805
Stock Repurchase Program, Authorized Amount	$ 300,000								300,000
Net Income	$ 22,546	$ 53,675	$ 49,510	$ 37,729	$ 47,348	$ (31,611)	$ 49,798	$ 36,053	$ 163,460	$ 101,588	$ 210,687
Weighted average common shares outstanding used in computing basic earnings per share									47,849	44,116	43,433
Common shares issuable upon exercise of dilutive stock options									512	543	626
Common shares issuable from contingently convertible debentures									0	3,674	6,314
Weighted average common shares outstanding and common share equivalents used in computing diluted earnings per share									48,361	48,333	50,373
Earnings Per Share attributable to WESCO International, Inc.
Basic	$ 0.48	$ 1.13	$ 1.03	$ 0.77	$ 0.97	$ (0.73)	$ 1.18	$ 0.85	$ 3.42	$ 2.30	$ 4.85
Diluted	$ 0.47	$ 1.12	$ 1.02	$ 0.76	$ 0.96	$ (0.73)	$ 1.02	$ 0.77	$ 3.38	$ 2.10	$ 4.18
Treasury Stock, Common [Member]
Accelerated Share Repurchases [Line Items]
Payments for Repurchase of Common Stock									$ 100,000		$ 150,000

EARNINGS PER SHARE - EARNINGS PER SHARE (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Payments for Repurchase of Common Stock	$ 106,792,000	$ 4,818,000	$ 155,805,000
Treasury Stock, Shares, Acquired	1,778,537		2,468,576
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities	0	3,674,000	6,314,000
Stock Appreciation Rights (SARs)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share	1,300,000	1,200,000
Convertible Debt Securities [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt Instrument, Convertible, Conversion Price	$ 28.87
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities	11,951,932
Dilutive Securities, Effect on Basic Earnings Per Share		$ 0.17	$ 0.60
Treasury Stock, Common [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Treasury Stock, Common, Shares		2,468,576
Payments for Repurchase of Common Stock	$ 100,000,000		$ 150,000,000
Treasury Stock, Shares, Acquired	1,778,537

Textuals (Details) - USD ($) $ in Millions	12 Months Ended
Textuals (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Employee Benefit Plans [Line Items]
Defined contribution plan, employer discretionary contribution amount	$ 10.0
Defined contribution plan, cost recognized	31.3	$ 18.5	$ 18.1
Deferred Compensation Liability, Classified, Noncurrent	24.3	$ 21.7
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 0.4
Defined Benefit Plan, Description	The Company sponsors a contributory defined benefit plan (the "Plan") covering substantially all Canadian employees of EECOL. The Plan provides retirement benefits based on earnings and credited service, and participants contribute 2% of their earnings to the Plan. Participants become 100% vested after two years of continuous service or, if earlier, at the participant's normal retirement age.
Defined Benefit Plan, Unfunded Plan	The Company also sponsors a Supplemental Executive Retirement Plan (the "SERP"), which provides additional pension benefits to certain executives of EECOL based on earnings, and credited service. Effective January 1, 2013, the SERP was closed to new participants and existing participants became 100% vested. SERP participants continue to contribute 4% of their earnings to the Plan.
Supplemental Employee Retirement Plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.50%	3.90%
United States
Schedule of Employee Benefit Plans [Line Items]
Defined contribution plan, employer matching contribution, percent	50.00%
Defined contribution plan, maximum annual contribution per employee, percent	6.00%
Canada \| Minimum
Schedule of Employee Benefit Plans [Line Items]
Defined contribution plan, employer matching contribution, percent	3.00%
Canada \| Maximum
Schedule of Employee Benefit Plans [Line Items]
Defined contribution plan, employer matching contribution, percent	5.00%

EMPLOYEE BENEFIT PLANS Benefit Obligations, Plan Assets, and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Accumulated Benefit Obligation (ABO) at December 31	$ 92,375	$ 75,666
PBO at beginning of year	96,160	87,186
Service cost	4,328	3,845	$ 4,537
Interest cost	3,912	3,856	4,012
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	735	709
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	5,562	5,328	5,260
Defined Benefit Plan, Amortization of Gain (Loss)	$ 149	$ 31	15
Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan [Table Text Block]	10906	2172
Defined Benefit Plan, Benefit Obligation, Benefits Paid	$ 3,005	$ 4,404
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	149	31	15
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax	8,742	2,787	(6,193)
PBO at end of year	120,319	96,160	87,186
Plan assets at beginning of year	84,753	79,185
Actual return on plan assets	7,875	4,115
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	735	709
Company contributions	368	1,956
Defined Benefit Plan, Plan Assets, Benefits Paid	3,005	4,404
Plan assets at end of year	97,182	84,753	$ 79,185
Funded status	(23,137)	(11,407)
Current liabilities	(395)	(364)
Non-current liabilities	(22,742)	(11,043)
Net pension liability at end of year	(23,137)	(11,407)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax	2,508	(6,234)
Defined Benefit Plan, Benefit Obligation, Foreign Currency Translation Gain (Loss)	7,283	2,796
Defined Benefit Plan, Plan Assets, Foreign Currency Translation Gain (Loss)	6,456	3,192
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax	$ 2,508	$ (6,234)

EMPLOYEE BENEFIT PLANS Pension Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Pension and Other Postretirement Benefit Expense	$ 8,910	$ 4,827	$ (1,258)
Service cost	4,328	3,845	4,537
Interest cost	3,912	3,856	4,012
Expected return on plan assets	(5,562)	(5,328)	(5,260)
Defined Benefit Plan, Amortization of Gain (Loss)	(149)	(31)	(15)
Total net periodic pension cost	2,529	2,342	3,274
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	8,593	2,756	(6,208)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	149	31	15
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax	8,742	2,787	(6,193)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	(2,361)	(302)	1,661
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	$ 6,381	$ 2,485	$ (4,532)

EMPLOYEE BENEFIT PLANS Assumptions (Details)	12 Months Ended
EMPLOYEE BENEFIT PLANS Assumptions (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	3.75%	4.00%	4.00%
Discount rate	3.50%	3.90%
Average salary increases	3.80%	3.80%
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	3.75%	4.00%	4.00%
Discount rate	3.50%	3.90%
Average salary increases	3.80%	3.80%

EMPLOYEE BENEFIT PLANS Assumptions 2 (Details)	12 Months Ended
EMPLOYEE BENEFIT PLANS Assumptions 2 (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.50%	3.90%
Discount rate	3.90%	4.20%	4.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	6.40%	6.40%	6.40%
Rate of compensation increase	3.75%	4.00%	4.00%
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.50%	3.90%
Discount rate	3.90%	4.20%	4.10%
Rate of compensation increase	3.75%	4.00%	4.00%

EMPLOYEE BENEFIT PLANS Benefit payments (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Retirement Benefits [Abstract]
2013	$ 3,092
2014	3,131
2015	3,215
2016	3,382
2017	3,488
2018-2023	$ 21,776

EMPLOYEE BENEFIT PLANS Asset allocations (Details)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure
Equity securities	100.00%	100.00%
Fixed Income Investments [Member]
Defined Benefit Plan Disclosure
Equity securities	41.40%	43.40%
Other Investments [Member]
Defined Benefit Plan Disclosure
Equity securities	21.70%	18.70%
Canada \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Equity securities	11.50%	11.70%
United States \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Equity securities	4.60%	4.60%
Non-US [Member] \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Equity securities	20.80%	21.60%

EMPLOYEE BENEFIT PLANS Target asset allocation (Details)	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan Disclosure
Defined Benefit Plan, Investment Policies and Strategies Narrative Description	The primary investment objective, in support of the overall objective, is to earn the highest rate of return possible for the Plan, while keeping risk at acceptable levels. The long-term return objective of the Plan is to achieve a minimum annualized rate of return in excess of the actuarial requirements. This translates into a required return of 3.0% above inflation, net of investment management fees. The return objective is consistent with the overall investment risk level that the Plan assumes in order to meet the pension obligations of the Plan. To achieve this long term investment objective, the Plan has adopted an asset mix that has a combination of primarily equity and fixed income investments. Risk is controlled by investing in a well-diversified portfolio of asset classes. A benchmark portfolio is established based on the expected returns for each asset class available. The investment of the Plan's assets in accordance with the benchmark portfolio should enable the Plan to not only attain, but also exceed the minimum overall objective.
Equity Securities [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Target Allocation Percentage of Assets, Equity Securities	40.00%
Fixed Income Funds [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Target Allocation Percentage of Assets, Equity Securities	45.00%
Other Investments [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Target Allocation Percentage of Assets, Equity Securities	15.00%
United States \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Target Allocation Percentage of Assets, Equity Securities	28.00%
Canada \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Target Allocation Percentage of Assets, Equity Securities	13.00%

EMPLOYEE BENEFIT PLANS Fair value plan assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure
Total investments	100.00%	100.00%
Defined Benefit Plan, Fair Value of Plan Assets	$ 97,182	$ 84,753	$ 79,185
Level 1
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	3,996	235
Level 2
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Level 3
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	$ 0	$ 0
Fixed Income Investments [Member]
Defined Benefit Plan Disclosure
Total investments	41.40%	43.40%
Defined Benefit Plan, Fair Value of Plan Assets	$ 40,193	$ 36,677
Fixed Income Investments [Member] \| Level 1
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Fixed Income Investments [Member] \| Level 2
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Fixed Income Investments [Member] \| Level 3
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	$ 0	$ 0
Other Investments [Member]
Defined Benefit Plan Disclosure
Total investments	21.70%	18.70%
Defined Benefit Plan, Fair Value of Plan Assets	$ 21,135	$ 15,983
Other Investments [Member] \| Level 1
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	3,996
Other Investments [Member] \| Level 2
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Other Investments [Member] \| Level 3
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	$ 0	$ 0
Canada \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Total investments	11.50%	11.70%
Defined Benefit Plan, Fair Value of Plan Assets	$ 11,211	$ 9,916
Canada \| Equity Securities [Member] \| Level 1
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Canada \| Equity Securities [Member] \| Level 2
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Canada \| Equity Securities [Member] \| Level 3
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	$ 0	$ 0
United States \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Total investments	4.60%	4.60%
Defined Benefit Plan, Fair Value of Plan Assets	$ 4,436	$ 3,881
United States \| Equity Securities [Member] \| Level 1
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
United States \| Equity Securities [Member] \| Level 2
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
United States \| Equity Securities [Member] \| Level 3
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	$ 0	$ 0
Non-US [Member] \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Total investments	20.80%	21.60%
Defined Benefit Plan, Fair Value of Plan Assets	$ 20,207	$ 18,296
Non-US [Member] \| Equity Securities [Member] \| Level 1
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Non-US [Member] \| Equity Securities [Member] \| Level 2
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Non-US [Member] \| Equity Securities [Member] \| Level 3
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Estimate of Fair Value Measurement [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	93,186	84,518
Estimate of Fair Value Measurement [Member] \| Fixed Income Investments [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	40,193	36,677
Estimate of Fair Value Measurement [Member] \| Other Investments [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	17,139	15,748
Estimate of Fair Value Measurement [Member] \| Canada \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	11,211	9,916
Estimate of Fair Value Measurement [Member] \| United States \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	4,436	3,881
Estimate of Fair Value Measurement [Member] \| Non-US [Member] \| Equity Securities [Member]
Defined Benefit Plan Disclosure
Defined Benefit Plan, Fair Value of Plan Assets	$ 20,207	$ 18,296

STOCK-BASED COMPENSATION - STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Increase to Number of Shares Authorized to be Issued				1,700,000
Stock-based compensation expense				$ 14,809	$ 12,493	$ 12,899
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value				$ 17,200	13,000	15,800
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized				3,400,000
Authorized Shares for Issuance Factor				Under the LTIP, the total number of shares of common stock authorized to be issued will be reduced by 1 share of common stock for every 1 share that is subject to a stock appreciation right granted, and 1.83 shares of common stock for every 1 share that is subject to an award other than a stock appreciation right granted on or after May 31, 2017.
Common Stock, Capital Shares Reserved for Future Issuance				3,600,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized				$ 18,700
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense				$ 6,400	$ 4,900	$ 5,700
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value				$ 21	$ 13	$ 22
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number				290,054	257,096	175,411	185,457
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value				$ 71.33	$ 44.45	$ 69.05
Selling, General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense				$ 14,800	$ 12,500	$ 12,900
Scenario, Forecast [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 800	$ 6,600	$ 11,300
Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number				74,254

STOCK-BASED COMPENSATION - SCHEDULE OF SHARE-BASED COMPENSATION, STOCK OPTIONS AND STOCK APPRECIATION RIGHTS AWARD ACTIVITY (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share Based Compensation Arrangements By Share Based Payment Award, Options and Stock Appreciation Rights Outstanding [Roll Forward]
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 42.19	$ 41.54	$ 35.80
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 76.63	$ 47.00	$ 67.81
Share Based Compensation Arrangements By Share Based Payment Award, Options and Stock Appreciation Rights Outstanding [Roll Forward]
Stock-settled appreciation rights granted	39,978	91,768	59,661
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 21	$ 13	$ 22
Share Based Compensation Arrangements By Share Based Payment Award, Options and Stock Appreciation Rights Outstanding [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Exercised	495,181	526,818	232,542
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Forfeitures	161,506	310,715	75,364
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 66.06	$ 63.71	$ 73.59
Weighted Average Remaining Contractual Term, Outstanding	6 years 2 months
Weighted Average Remaining Contractual Term, Exercisable	4 years 7 months
Aggregate Intrinsic Value, Outstanding	$ 28,791,000
Aggregate Intrinsic Value, Exercisable	$ 18,801,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number	1,331,580	1,549,350	2,034,263
Stock-settled appreciation rights granted	455,807	709,999	394,182
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price	$ 56.96	$ 53.35	$ 49.36
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 57.75	$ 52.62	$ 54.47	$ 50.91
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	2,238,607	2,439,487	2,567,021	2,480,745
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	$ 71.21	$ 42.63	$ 69.54

STOCK-BASED COMPENSATION - SUMMARY OF RESTRICTED STOCK UNITS AND PERFORMANCE-BASED AWARDS (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Unvested, Shares	148,508	149,320	114,520	130,004
Granted	39,978	91,768	59,661
Vested in Period	0	0	(38,869)
Forfeited in Period	(40,790)	(56,968)	(36,276)
Unvested, Weighted Average Fair Value	$ 60.23	$ 60.36	$ 76.48	$ 80.21
Granted, Weighted Average Fair Value	76.63	47.00	67.81
Vested in Period, Weighted Average Fair Value	0.00	0.00	72.25
Forfeited in Period, Weighted Average Fair Value	$ 76.77	$ 71.25	$ 80.14
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Unvested, Shares	290,054	257,096	175,411	185,457
Granted	100,993	162,256	81,022
Vested in Period	(44,720)	(60,015)	(76,387)
Forfeited in Period	(23,315)	(20,556)	(14,681)
Unvested, Weighted Average Fair Value	$ 58.11	$ 57.47	$ 74.52	$ 73.87
Granted, Weighted Average Fair Value	71.33	44.45	69.05
Vested in Period, Weighted Average Fair Value	84.57	72.41	66.89
Forfeited in Period, Weighted Average Fair Value	$ 57.52	$ 59.15	$ 75.73
Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Unvested, Shares	74,254

STOCK-BASED COMPENSATION - SCHEDULE OF SHARE-BASED PAYMENT AWARDS, VALUATION ASSUMPTIONS (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk free interest rate	1.90%	1.20%	1.60%
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-settled appreciation rights granted	455,807	709,999	394,182
Expected life (in years)	5 years	5 years	5 years
Expected volatility	28.60%	32.20%	32.30%
Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	74,254

STOCK-BASED COMPENSATION - SCHEDULE OF SHARE-BASED PAYMENT AWARD, PERFORMANCE-BASED AWARDS, VALUATION ASSUMPTIONS (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.90%	1.20%	1.60%
Correlation	0.00%	0.00%	0.00%
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grant date share price	$ 71.65	$ 42.44	$ 69.54
WESCO expected volatility	0.00%	0.00%	0.00%
Risk-free interest rate	0.00%	0.00%	0.00%

COMMITMENTS AND CONTINGENCIES - COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Operating Leases, Rent Expense	$ 82.0	$ 76.7	$ 70.7

COMMITMENTS AND CONTINGENCIES - SCHEDULE OF FUTURE MINIMUM RENTAL PAYMENTS (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Operating Leases, Future Minimum Payments Due [Abstract]
2013	$ 65,510
2014	55,378
2015	44,036
2016	33,498
2017	23,904
Thereafter	$ 49,150

SEGMENTS AND RELATED INFORMATION - SEGMENTS AND RELATED INFORMATION (Details)	Dec. 31, 2017 stockkeepingunits products
Segment Reporting [Abstract]
Product stock keeping units (in stock keeping units) \| stockkeepingunits	230,000
Products (in products) \| products	1,000,000

SEGMENTS AND RELATED INFORMATION - SCHEDULE OF REVENUE FROM EXTERNAL CUSTOMERS AND LONG LIVED ASSETS, BY GEOGRAPHICAL AREAS (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 1,996,647	$ 2,000,159	$ 1,909,624	$ 1,772,591	$ 1,793,262	$ 1,855,212	$ 1,911,582	$ 1,775,961	$ 7,679,021	$ 7,336,017	$ 7,518,487

OTHER FINANCIAL INFORMATION - OTHER FINANCIAL INFORMATION (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 1,360	$ 1,400
Unsecured Debt [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	500
Unsecured Debt [Member] \| 2024 Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 350

OTHER FINANCIAL INFORMATION - CONDENSED CONSOLIDATED BALANCE SHEETS (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Information
Cash and cash equivalents	$ 117,953	$ 110,131	$ 160,279	$ 128,319
Trade accounts receivable	1,170,080	1,034,402
Inventories, net	956,148	821,441
Prepaid Expense and Other Assets, Current	40,189	46,360
Total current assets	2,408,849	2,103,078
Property, building and equipment, net	156,445	157,607
Intangible assets, net	367,104	393,362
Goodwill and other intangibles, net	1,771,877	1,730,950	1,681,662
Other assets	6,990	31,041
Total assets	4,735,468	4,431,841
Accounts payable	799,520	684,721
Short-term debt	34,075	20,920
Other Liabilities, Current	86,108	78,425
Total current liabilities	1,040,969	873,799
Long-term debt	1,313,261	1,363,135
Long-term Debt, Excluding Current Maturities	1,313,261	1,363,135
Total WESCO International stockholders' equity	2,116,143	1,963,631
Stockholders' Equity Attributable to Parent	2,119,739	1,966,900
Noncontrolling interest	(3,596)	(3,269)		(487)
Total liabilities and stockholders' equity	4,735,468	4,431,841
Parent Company
Condensed Financial Information
Cash and cash equivalents	0	0	0	0
Trade accounts receivable	0	0
Inventories, net	0	0
Prepaid Expense and Other Assets, Current	4,730	13,647
Total current assets	4,730	13,647
Intercompany receivables, net	0	0
Property, building and equipment, net	0	0
Intangible assets, net	0	0
Goodwill and other intangibles, net	0	0
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	3,058,613	3,538,476
Other assets	0	0
Total assets	3,063,343	3,552,123
Accounts payable	0	0
Short-term debt	0	0
Other Liabilities, Current	0	0
Total current liabilities	0	0
Intercompany payables, net	939,784	1,572,486
Long-term Debt, Excluding Current Maturities	0	0
Liabilities, Other than Long-term Debt, Noncurrent	3,820	12,737
Stockholders' Equity Attributable to Parent	2,119,739	1,966,900
Noncontrolling interest	0	0
Total liabilities and stockholders' equity	3,063,343	3,552,123
Guarantor Subsidiaries
Condensed Financial Information
Cash and cash equivalents	50,602	41,552	38,963	32,508
Trade accounts receivable	0	0
Inventories, net	430,092	364,562
Prepaid Expense and Other Assets, Current	42,547	34,833
Total current assets	523,241	440,947
Intercompany receivables, net	0	0
Property, building and equipment, net	50,198	51,824
Intangible assets, net	2,770	3,417
Goodwill and other intangibles, net	257,623	257,623
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	5,023,826	4,028,502
Other assets	2,778	23,846
Total assets	5,860,436	4,806,159
Accounts payable	417,690	381,795
Short-term debt	0	0
Other Liabilities, Current	80,039	120,299
Total current liabilities	497,729	502,094
Intercompany payables, net	1,249,352	487,850
Long-term Debt, Excluding Current Maturities	934,033	983,449
Liabilities, Other than Long-term Debt, Noncurrent	120,709	55,898
Stockholders' Equity Attributable to Parent	3,058,613	2,776,868
Noncontrolling interest	0	0
Total liabilities and stockholders' equity	5,860,436	4,806,159
Non-Guarantor Subsidiaries
Condensed Financial Information
Cash and cash equivalents	67,351	68,579	121,316	95,811
Trade accounts receivable	1,170,080	1,034,402
Inventories, net	526,056	456,879
Prepaid Expense and Other Assets, Current	152,531	211,637
Total current assets	1,916,018	1,771,497
Intercompany receivables, net	2,189,136	2,060,336
Property, building and equipment, net	106,247	105,783
Intangible assets, net	364,334	389,945
Goodwill and other intangibles, net	1,514,254	1,473,327
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0	0
Other assets	28,415	22,998
Total assets	6,118,404	5,823,886
Accounts payable	381,830	302,926
Short-term debt	34,075	20,920
Other Liabilities, Current	162,475	170,872
Total current liabilities	578,380	494,718
Intercompany payables, net	0	0
Long-term Debt, Excluding Current Maturities	379,228	379,686
Liabilities, Other than Long-term Debt, Noncurrent	140,566	162,641
Stockholders' Equity Attributable to Parent	5,023,826	4,790,110
Noncontrolling interest	(3,596)	(3,269)
Total liabilities and stockholders' equity	6,118,404	5,823,886
Consolidated Entities [Member]
Condensed Financial Information
Cash and cash equivalents	117,953	110,131	160,279	128,319
Trade accounts receivable	1,170,080	1,034,402
Inventories, net	956,148	821,441
Prepaid Expense and Other Assets, Current	164,668	137,104
Total current assets	2,408,849	2,103,078
Intercompany receivables, net	0	0
Property, building and equipment, net	156,445	157,607
Intangible assets, net	367,104	393,362
Goodwill and other intangibles, net	1,771,877	1,730,950
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0	0
Other assets	31,193	46,844
Total assets	4,735,468	4,431,841
Accounts payable	799,520	684,721
Short-term debt	34,075	20,920
Other Liabilities, Current	207,374	168,158
Total current liabilities	1,040,969	873,799
Intercompany payables, net	0	0
Long-term Debt, Excluding Current Maturities	1,313,261	1,363,135
Liabilities, Other than Long-term Debt, Noncurrent	265,095	231,276
Stockholders' Equity Attributable to Parent	2,119,739	1,966,900
Noncontrolling interest	(3,596)	(3,269)
Total liabilities and stockholders' equity	4,735,468	4,431,841
Consolidation, Eliminations
Condensed Financial Information
Cash and cash equivalents	0	0	$ 0	$ 0
Trade accounts receivable	0	0
Inventories, net	0	0
Prepaid Expense and Other Assets, Current	(35,140)	(123,013)
Total current assets	(35,140)	(123,013)
Intercompany receivables, net	(2,189,136)	(2,060,336)
Property, building and equipment, net	0	0
Intangible assets, net	0	0
Goodwill and other intangibles, net	0	0
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	(8,082,439)	(7,566,978)
Other assets	0	0
Total assets	(10,306,715)	(9,750,327)
Accounts payable	0	0
Short-term debt	0	0
Other Liabilities, Current	(35,140)	(123,013)
Total current liabilities	(35,140)	(123,013)
Intercompany payables, net	(2,189,136)	(2,060,336)
Long-term Debt, Excluding Current Maturities	0	0
Liabilities, Other than Long-term Debt, Noncurrent	0	0
Stockholders' Equity Attributable to Parent	(8,082,439)	(7,566,978)
Noncontrolling interest	0	0
Total liabilities and stockholders' equity	$ (10,306,715)	$ (9,750,327)

OTHER FINANCIAL INFORMATION - CONDENSED CONSOLIDATED INCOME STATEMENT (Details) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Information
Net sales	$ 1,996,647	$ 2,000,159	$ 1,909,624	$ 1,772,591	$ 1,793,262	$ 1,855,212	$ 1,911,582	$ 1,775,961		$ 7,679,021	$ 7,336,017	$ 7,518,487
Cost of goods sold	1,613,469	1,614,814	1,543,510	1,422,573	1,444,735	1,490,173	1,532,113	1,420,793		6,194,366	5,887,814	6,024,826
Selling, General and Administrative Expense										(1,099,748)	(1,049,286)	(1,054,951)
Depreciation, Depletion and Amortization, Nonproduction										(64,017)	(66,858)	(64,968)
Loss on debt extinguishment						124,000			$ (123,900)	0	123,933	0
Income tax expense										89,307	30,431	95,537
Net income	22,222	53,576	49,535	37,800	47,755	(31,021)	49,852	34,534		163,133	101,120	208,373
Net Income (Loss) Attributable to Noncontrolling Interest										(327)	(468)	(2,314)
Net Income	$ 22,546	$ 53,675	$ 49,510	$ 37,729	$ 47,348	$ (31,611)	$ 49,798	$ 36,053		163,460	101,588	210,687
Foreign currency translation adjustment										85,762	38,275	(225,795)
Benefit plan adjustments										(6,381)	(2,485)	4,532
Comprehensive Income (Loss), Net of Tax, Attributable to Parent										242,841	137,378	(10,576)
Parent Company
Condensed Financial Information
Net sales										0	0	0
Cost of goods sold										0	0	0
Selling, General and Administrative Expense										0	(61)	(26)
Depreciation, Depletion and Amortization, Nonproduction										0	0	0
Income (Loss) from Subsidiaries, before Tax										160,587	240,571	225,370
Interest Income (Expense), Net										0	17,555	24,910
Loss on debt extinguishment											(123,933)
Income tax expense										(2,546)	(2,098)	(7,939)
Net income										163,133	101,120	208,373
Net Income (Loss) Attributable to Noncontrolling Interest										0	0	0
Net Income										163,133	101,120	208,373
Foreign currency translation adjustment										85,762	38,275	(225,795)
Benefit plan adjustments										(6,381)	(2,485)	4,532
Comprehensive Income (Loss), Net of Tax, Attributable to Parent										242,514	136,910	(12,890)
Guarantor Subsidiaries
Condensed Financial Information
Net sales										3,370,088	3,306,265	3,456,883
Cost of goods sold										2,714,511	2,651,409	2,784,413
Selling, General and Administrative Expense										(555,503)	(477,437)	(611,549)
Depreciation, Depletion and Amortization, Nonproduction										(18,442)	(20,226)	(19,703)
Income (Loss) from Subsidiaries, before Tax										168,782	155,814	219,619
Interest Income (Expense), Net										94,313	87,824	63,261
Loss on debt extinguishment											0
Income tax expense										(4,486)	8,263	(6,929)
Net income										160,587	216,920	204,505
Net Income (Loss) Attributable to Noncontrolling Interest										0	0	0
Net Income										160,587	216,920	204,505
Foreign currency translation adjustment										85,762	38,275	(225,795)
Benefit plan adjustments										(6,381)	(2,485)	4,532
Comprehensive Income (Loss), Net of Tax, Attributable to Parent										239,968	252,710	(16,758)
Non-Guarantor Subsidiaries
Condensed Financial Information
Net sales										4,441,655	4,134,508	4,177,383
Cost of goods sold										3,612,577	3,341,161	3,356,192
Selling, General and Administrative Expense										(544,245)	(571,788)	(443,376)
Depreciation, Depletion and Amortization, Nonproduction										(45,575)	(46,632)	(45,265)
Income (Loss) from Subsidiaries, before Tax										0	0	0
Interest Income (Expense), Net										(25,863)	(28,804)	(18,339)
Loss on debt extinguishment											0
Income tax expense										96,339	24,266	110,405
Net income										168,782	179,465	240,484
Net Income (Loss) Attributable to Noncontrolling Interest										(327)	(468)	(2,314)
Net Income										169,109	179,933	242,798
Foreign currency translation adjustment										85,762	38,275	(225,795)
Benefit plan adjustments										(6,381)	(2,485)	4,532
Comprehensive Income (Loss), Net of Tax, Attributable to Parent										248,490	215,723	21,535
Consolidated Entities [Member]
Condensed Financial Information
Net sales										7,679,021	7,336,017	7,518,487
Cost of goods sold										6,194,366	5,887,814	6,024,826
Selling, General and Administrative Expense										(1,099,748)	(1,049,286)	(1,054,951)
Depreciation, Depletion and Amortization, Nonproduction										(64,017)	(66,858)	(64,968)
Income (Loss) from Subsidiaries, before Tax										0	0	0
Interest Income (Expense), Net										68,450	76,575	69,832
Loss on debt extinguishment											(123,933)
Income tax expense										89,307	30,431	95,537
Net income										163,133	101,120	208,373
Net Income (Loss) Attributable to Noncontrolling Interest										(327)	(468)	(2,314)
Net Income										163,460	101,588	210,687
Foreign currency translation adjustment										85,762	38,275	(225,795)
Benefit plan adjustments										(6,381)	(2,485)	4,532
Comprehensive Income (Loss), Net of Tax, Attributable to Parent										242,841	137,378	(10,576)
Consolidation, Eliminations
Condensed Financial Information
Net sales										(132,722)	(104,756)	(115,779)
Cost of goods sold										(132,722)	(104,756)	(115,779)
Selling, General and Administrative Expense										0	0	0
Depreciation, Depletion and Amortization, Nonproduction										0	0	0
Income (Loss) from Subsidiaries, before Tax										(329,369)	(396,385)	(444,989)
Interest Income (Expense), Net										0	0	0
Loss on debt extinguishment											0
Income tax expense										0	0	0
Net income										(329,369)	(396,385)	(444,989)
Net Income (Loss) Attributable to Noncontrolling Interest										0	0	0
Net Income										(329,369)	(396,385)	(444,989)
Foreign currency translation adjustment										(171,524)	(76,550)	451,590
Benefit plan adjustments										12,762	4,970	(9,064)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent										$ (488,131)	$ (467,965)	$ (2,463)

OTHER FINANCIAL INFORMATION - CONDENSED CONSOLIDATED STATEMENT OF CASH FLOWS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Information
Net cash (used) provided by operating activities	$ 149,122	$ 300,235	$ 283,049
Investing Activities:
Capital expenditures	(21,507)	(17,957)	(21,658)
Payments to Acquire Businesses, Net of Cash Acquired	0	(50,890)	(151,595)
Proceeds from sale of assets	6,766	8,361	3,023
Advances to subsidiaries and other	9,446	(10,000)	0
Net cash used in investing activities	(5,295)	(70,486)	(170,230)
Financing Activities:
Other	6,722	(4,570)	(2,089)
Net cash provided (used) by financing activities	(141,196)	(276,263)	(67,815)
Effect of exchange rate changes on cash and cash equivalents	5,191	(3,634)	(13,044)
Net change in cash and cash equivalents	7,822	(50,148)	31,960
Cash and cash equivalents at the beginning of period	110,131	160,279	128,319
Cash and cash equivalents at the end of period	117,953	110,131	160,279
Parent Company
Condensed Financial Information
Net cash (used) provided by operating activities	(36,575)	95,388	3,531
Investing Activities:
Capital expenditures	0	0	0
Payments to Acquire Businesses, Net of Cash Acquired		0	0
Proceeds from sale of assets	0	0	0
Proceeds from Dividends Received	0	0	0
Payments for Advance to Affiliate	0	0	0
Net cash used in investing activities	0	0	0
Financing Activities:
Proceeds from Issuance of Debt	143,367	252,246	150,705
Repayments of Debt	0	(344,804)	0
Equity transactions	(106,792)	(2,830)	(154,236)
Cash Dividends Paid to Parent Company	0	0	0
Other	0	0	0
Net cash provided (used) by financing activities	36,575	(95,388)	(3,531)
Effect of exchange rate changes on cash and cash equivalents	0	0	0
Net change in cash and cash equivalents	0	0	0
Cash and cash equivalents at the beginning of period	0	0	0
Cash and cash equivalents at the end of period	0	0	0
Guarantor Subsidiaries
Condensed Financial Information
Net cash (used) provided by operating activities	101,826	(243,476)	214,037
Investing Activities:
Capital expenditures	(13,215)	(12,482)	(15,266)
Payments to Acquire Businesses, Net of Cash Acquired		(50,890)	(151,595)
Proceeds from sale of assets	0	0	0
Proceeds from Dividends Received	307,784	82,912	114,101
Payments for Advance to Affiliate	383,686	297,259	197,345
Net cash used in investing activities	(89,117)	(277,719)	(250,105)
Financing Activities:
Proceeds from Issuance of Debt	775,926	1,566,864	1,224,596
Repayments of Debt	(785,392)	(1,030,520)	(1,175,056)
Equity transactions	0	0	0
Cash Dividends Paid to Parent Company	0	0	0
Other	5,807	(12,560)	(7,017)
Net cash provided (used) by financing activities	(3,659)	523,784	42,523
Effect of exchange rate changes on cash and cash equivalents	0	0	0
Net change in cash and cash equivalents	9,050	2,589	6,455
Cash and cash equivalents at the beginning of period	41,552	38,963	32,508
Cash and cash equivalents at the end of period	50,602	41,552	38,963
Non-Guarantor Subsidiaries
Condensed Financial Information
Net cash (used) provided by operating activities	83,871	448,323	65,481
Investing Activities:
Capital expenditures	(8,292)	(5,475)	(6,392)
Payments to Acquire Businesses, Net of Cash Acquired		0	0
Proceeds from sale of assets	6,766	8,361	3,023
Proceeds from Dividends Received	0	0	0
Payments for Advance to Affiliate	(26,912)	337,344	(17,461)
Net cash used in investing activities	25,386	(334,458)	14,092
Financing Activities:
Proceeds from Issuance of Debt	1,144,848	672,345	452,655
Repayments of Debt	(952,740)	(752,401)	(379,578)
Equity transactions	0	0	0
Cash Dividends Paid to Parent Company	(307,784)	(82,912)	(114,101)
Other	0	0	0
Net cash provided (used) by financing activities	(115,676)	(162,968)	(41,024)
Effect of exchange rate changes on cash and cash equivalents	5,191	(3,634)	(13,044)
Net change in cash and cash equivalents	(1,228)	(52,737)	25,505
Cash and cash equivalents at the beginning of period	68,579	121,316	95,811
Cash and cash equivalents at the end of period	67,351	68,579	121,316
Consolidated Entities [Member]
Condensed Financial Information
Net cash (used) provided by operating activities	149,122	300,235	283,049
Investing Activities:
Capital expenditures	(21,507)	(17,957)	(21,658)
Payments to Acquire Businesses, Net of Cash Acquired		(50,890)	(151,595)
Proceeds from sale of assets	6,766	8,361	3,023
Proceeds from Dividends Received	0	0	0
Payments for Advance to Affiliate	(9,446)	10,000	0
Net cash used in investing activities	(5,295)	(70,486)	(170,230)
Financing Activities:
Proceeds from Issuance of Debt	1,680,455	2,194,196	1,630,611
Repayments of Debt	(1,720,666)	(2,455,069)	(1,537,173)
Equity transactions	(106,792)	(2,830)	(154,236)
Cash Dividends Paid to Parent Company	0	0	0
Other	5,807	(12,560)	(7,017)
Net cash provided (used) by financing activities	(141,196)	(276,263)	(67,815)
Effect of exchange rate changes on cash and cash equivalents	5,191	(3,634)	(13,044)
Net change in cash and cash equivalents	7,822	(50,148)	31,960
Cash and cash equivalents at the beginning of period	110,131	160,279	128,319
Cash and cash equivalents at the end of period	117,953	110,131	160,279
Consolidation, Eliminations
Condensed Financial Information
Net cash (used) provided by operating activities	0	0	0
Investing Activities:
Capital expenditures	0	0	0
Payments to Acquire Businesses, Net of Cash Acquired		0	0
Proceeds from sale of assets	0	0	0
Proceeds from Dividends Received	(307,784)	(82,912)	(114,101)
Payments for Advance to Affiliate	(366,220)	(624,603)	(179,884)
Net cash used in investing activities	58,436	541,691	65,783
Financing Activities:
Proceeds from Issuance of Debt	(383,686)	(297,259)	(197,345)
Repayments of Debt	17,466	(327,344)	17,461
Equity transactions	0	0	0
Cash Dividends Paid to Parent Company	307,784	82,912	114,101
Other	0	0	0
Net cash provided (used) by financing activities	(58,436)	(541,691)	(65,783)
Effect of exchange rate changes on cash and cash equivalents	0	0	0
Net change in cash and cash equivalents	0	0	0
Cash and cash equivalents at the beginning of period	0	0	0
Cash and cash equivalents at the end of period	$ 0	$ 0	$ 0

SELECTED QUARTERLY FINANCIAL DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Gain (Loss) on Extinguishment of Debt						$ (124,000)			$ 123,900	$ 0	$ (123,933)	$ 0
Net sales	$ 1,996,647	$ 2,000,159	$ 1,909,624	$ 1,772,591	$ 1,793,262	1,855,212	$ 1,911,582	$ 1,775,961		7,679,021	7,336,017	7,518,487
Cost of goods sold	1,613,469	1,614,814	1,543,510	1,422,573	1,444,735	1,490,173	1,532,113	1,420,793		6,194,366	5,887,814	6,024,826
Income from operations	81,446	89,250	83,105	67,089	82,009	92,555	87,987	69,508		320,890	332,059	373,742
Income before income taxes	63,844	71,939	66,289	50,368	64,507	(52,170)	68,535	50,679		252,440	131,551	303,910
Net income	22,222	53,576	49,535	37,800	47,755	(31,021)	49,852	34,534		163,133	101,120	208,373
Net Income attributable to WESCO International, Inc.	$ 22,546	$ 53,675	$ 49,510	$ 37,729	$ 47,348	$ (31,611)	$ 49,798	$ 36,053		$ 163,460	$ 101,588	$ 210,687
Basic earnings per share attributable to WESCO International, Inc.	$ 0.48	$ 1.13	$ 1.03	$ 0.77	$ 0.97	$ (0.73)	$ 1.18	$ 0.85		$ 3.42	$ 2.30	$ 4.85
Diluted earnings per share attributable to WESCO International, Inc.	$ 0.47	$ 1.12	$ 1.02	$ 0.76	$ 0.96	$ (0.73)	$ 1.02	$ 0.77		$ 3.38	$ 2.10	$ 4.18

Schedule II - Valuation and Qualifying Accounts (Details) - Allowance for Doubtful Accounts - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Dec. 31, 2014

Movement in Valuation Allowances and Reserves [Roll Forward]

Valuation Allowances and Reserves, Balance

$ 21,313

$ 22,007

$ 22,587

$ 21,084

Valuation Allowances and Reserves, Charged to Cost and Expense

8,466

5,888

6,099

Valuation Allowances and Reserves, Charged to Other Accounts

1,305

Valuation Allowances and Reserves, Deductions

^[1]

$ (9,160)

$ (6,489)

$ (5,901)

[1]	(1) Represents allowance for doubtful accounts in connection with certain acquisitions and divestitures.

Label	Element	Value
Stock Granted, Value, Share-based Compensation, Net of Forfeitures	us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation	$ 12,899,000
Stock Granted, Value, Share-based Compensation, Net of Forfeitures	us-gaap_StockGrantedDuringPeriodValueSharebasedCompensation	12,493,000
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options	us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationAndExerciseOfStockOptions	1,344,000
Adjustments to Additional Paid in Capital, Share-based Compensation and Exercise of Stock Options	us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationAndExerciseOfStockOptions	(2,876,000)
Treasury Stock, Value, Acquired, Cost Method	us-gaap_TreasuryStockValueAcquiredCostMethod	153,013,000
Additional Paid-in Capital [Member]
Treasury Stock, Value, Acquired, Par Value Method	us-gaap_TreasuryStockValueAcquiredParValueMethod	3,013,000
Adjustments Related to Tax Withholding for Share-based Compensation	us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	(2,202,000)
Adjustments Related to Tax Withholding for Share-based Compensation	us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	(1,253,000)
AOCI Attributable to Parent [Member]
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	(225,795,000)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax	38,275,000
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	4,532,000
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	$ (2,485,000)
Treasury Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	44,267
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	44,191
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	$ (3,300,000)
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	(3,224,000)
Stock Issued During Period, Value, Conversion of Convertible Securities, Net of Adjustments	us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecuritiesNetOfAdjustments	$ 233,366,000
Stock Issued During Period, Shares, Conversion of Convertible Securities	us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities	6,261,497
Noncontrolling Interest [Member]
Net Income (Loss) Attributable to Noncontrolling Interest	us-gaap_NetIncomeLossAttributableToNoncontrollingInterest	$ 2,314,000
Net Income (Loss) Attributable to Noncontrolling Interest	us-gaap_NetIncomeLossAttributableToNoncontrollingInterest	(468,000)
Retained Earnings [Member]
Adjustments Related to Tax Withholding for Share-based Compensation	us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	(145,000)
Adjustments Related to Tax Withholding for Share-based Compensation	us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	488,000
Common Stock [Member]
Stock Issued During Period, Value, Conversion of Convertible Securities	us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities	0
Stock Issued During Period, Value, Conversion of Convertible Securities	us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	230,206
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	230,464
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	$ 2,000
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	$ 2,000
Stock Issued During Period, Shares, Conversion of Convertible Securities	us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities	427
Stock Issued During Period, Shares, Conversion of Convertible Securities	us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities	7,295
Shares Paid for Tax Withholding for Share Based Compensation	us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation	33,989
Shares Paid for Tax Withholding for Share Based Compensation	us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation	17,358
Adjustments Related to Tax Withholding for Share-based Compensation	us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	$ 0
Adjustments Related to Tax Withholding for Share-based Compensation	us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation	$ 0