ACCOUNTING POLICIES - GENERAL (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016
Defined Benefit Plan Disclosure
Operating Lease, Right-of-Use Asset	$ 235,834	$ 0		$ 240,000
Income Taxes Receivable, Current	16,371	24,873
Prepaid Expense and Other Assets, Current	76,076	52,107
Assets, Current	2,540,411	2,385,640
Intangible Assets, Net (Including Goodwill)	$ 1,900,000	1,800,000
Characteristics of Securitizations or Asset-backed Financing Arrangements that are Accounted for as Sale	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. Since WESCO maintains control of the transferred receivables, 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. 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.
Self Insurance Reserve	$ 12,900	13,100
Accounts Receivable, Allowance for Credit Loss, Current	25,400	24,500
Accounts Receivable, Credit Loss Expense (Reversal)	7,000	10,900	$ 8,500
Inventory Valuation Reserves	30,700	27,600
Inventory Write-down	10,000	9,700	8,800
General and Administrative Costs in Inventory, Amount Incurred	71,200	69,200
Stock-based compensation expense	19,062	16,445	14,809
Goodwill	1,759,040	1,722,603	1,771,877
Assets	5,017,635	4,605,036
Liabilities, Current	1,084,059	1,061,946
Deferred Tax Liabilities, Net, Noncurrent	146,617	143,967
Liabilities	2,758,964	2,475,310
Retained Earnings (Accumulated Deficit)	2,530,429	2,307,462	2,079,697		$ 1,914,757
Accumulated Other Comprehensive Income (Loss), Net of Tax	(367,772)	(408,435)	$ (312,590)		$ (391,971)
Stockholders' Equity Attributable to Parent	2,265,483	2,135,310
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,258,671	2,129,726
Liabilities and Equity	5,017,635	$ 4,605,036
Operating Lease, Liability	$ 241,876			$ 245,000
Pension Plan [Member]
Defined Benefit Plan Disclosure
Discount rate	3.20%	4.00%
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