ACQUISITIONS (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) countries $ / shares shares	Dec. 31, 2020 USD ($) countries numberOfCities $ / shares shares	Dec. 31, 2019 USD ($) shares	Jun. 22, 2020 USD ($) $ / shares shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares
Business Acquisition [Line Items]
Goodwill, Acquired During Period		$ 1,384,027,000	$ 5,767,000
Preferred stock, shares issued | shares	21,612	21,612	0		0	0
Preferred stock, par value | $ / shares	$ 0.01	$ 0.01
Business Acquisition, Description of Acquired Entity		Anixter is a leading distributor of network and security solutions, electrical and electronic solutions, and utility power solutions with locations in over 300 cities across approximately 50 countries, and 2019 annual sales of more than $8 billion.
Business Combination, Reason for Business Combination		The Merger brought together two companies with highly compatible capabilities and characteristics. The combination of WESCO and Anixter created an enterprise with scale and should afford the Company the opportunity to digitize its business, and expand its services portfolio and supply chain offerings.
Number of Cities in which an Entity Operates | numberOfCities		300
Business Combination, Provisional Information, Initial Accounting Incomplete, Reasons		The estimated fair values of assets acquired and liabilities assumed are based on preliminary calculations and valuations using estimates and assumptions at the time of acquisition. The determination of the fair values of assets acquired and liabilities assumed, especially those related to identifiable intangible assets, is preliminary due to the complexity of combining multibillion dollar businesses. Accordingly, as the Company obtains additional information during the measurement period (not to exceed one year from the acquisition date), estimates and assumptions for the preliminary purchase consideration allocations may change materially.
Anixter International [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Jun. 22, 2020
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period		$ 8,000,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill				$ 1,838,065,000
Payments to Acquire Businesses, Gross		$ 3,811,038,000
Common stock, shares issued | shares				0.2397
Preferred stock, shares issued | shares				0.6356
Common Stock				$ 0.01
Preferred Stock, Dividend Rate, Percentage		10.625%
Preferred stock, par value | $ / shares				$ 25,000
Additional countries (in countries) | countries	50	50
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual		$ 4,500,000,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual		180,000,000.0
Business Acquisition, Transaction Costs	$ 132,200,000	132,200,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory		8,200,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Operating Lease Assets		18,000,000.0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles	$ 6,400,000	5,400,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Noncurrent Assets		15,500,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Operating Lease Liabilities		17,000,000.0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes		27,500,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Noncurrent Liabilities		38,700,000
Goodwill, Purchase Accounting Adjustments		2,400,000
Business Acquisition, Pro Forma Revenue		$ 16,016,902,000	$ 17,204,472,000
Anixter International [Member] | Cash [Member]
Business Acquisition [Line Items]
Business Acquisition, Share Price | $ / shares				$ 72.82
Sylvania Lighting Services Corp. [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Mar. 05, 2019
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period		$ 100,000,000
Noncompete Agreements [Member] | Anixter International [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		2 years
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
Maximum [Member] | Customer Relationships
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		20 years
Maximum [Member] | Noncompete Agreements [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		5 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		10 years
Minimum | Customer Relationships
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		10 years
Minimum | Noncompete Agreements [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		2 years