EARNINGS PER SHARE - SCHEDULE OF EARNINGS PER SHARE, BASIC AND DILUTED (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014
Accelerated Share Repurchases [Line Items]
Payments for Repurchase of Common Stock									$ 153,049	$ 127,169	$ 106,792
Stock Repurchase Program, Authorized Amount	$ 400,000				$ 300,000				$ 400,000	$ 300,000	$ 300,000	$ 300,000
Stock Repurchase Program Expiration Date									Dec. 31, 2020		Dec. 31, 2017
Treasury Stock, Shares, Acquired									3,455,584	2,003,446	1,778,537
Net Income	$ 53,098	$ 64,495	$ 63,464	$ 42,369	$ 58,133	$ 66,849	$ 57,940	$ 44,421	$ 223,426	$ 227,343	$ 163,460
Weighted average common shares outstanding used in computing basic earnings per share									43,104,000	46,722,000	47,849,000
Common shares issuable upon exercise of dilutive stock options									383,000	477,000	512,000
Weighted average common shares outstanding and common share equivalents used in computing diluted earnings per share									43,487,000	47,199,000	48,361,000
Earnings Per Share attributable to WESCO International, Inc.
Basic	$ 1.27	$ 1.53	$ 1.46	$ 0.94	$ 1.27	$ 1.42	$ 1.23	$ 0.94	$ 5.18	$ 4.87	$ 3.42
Diluted	$ 1.26	$ 1.52	$ 1.45	$ 0.93	$ 1.26	$ 1.41	$ 1.22	$ 0.93	$ 5.14	$ 4.82	$ 3.38
Treasury Stock, Common [Member]
Accelerated Share Repurchases [Line Items]
Payments for Repurchase of Common Stock			$ 150,000						$ 275,000	$ 125,000	$ 100,000
Treasury Stock, Common, Shares	5,459,030			365,272					5,459,030
Treasury Stock, Shares, Acquired									3,090,312	2,368,738	1,778,537