OTHER FINANCIAL INFORMATION - CONDENSED CONSOLIDATED BALANCE SHEETS (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Information
Cash and cash equivalents	$ 150,902		$ 96,343	$ 117,953	$ 110,131
Trade accounts receivable	1,187,359		1,166,607
Inventories, net	1,011,674		948,726
Prepaid Expense and Other Assets, Current	76,076		52,107
Total current assets	2,540,411		2,385,640
Property, building and equipment, net	181,448		160,878
Operating Lease, Right-of-Use Asset	235,834	$ 240,000	0
Intangible assets, net	287,275		316,016
Goodwill and other intangibles, net	1,759,040		1,722,603	1,771,877
Other assets	2,379		3,525
Total assets	5,017,635		4,605,036
Accounts payable	830,478		794,348
Short-term debt	26,255		30,785
Other Liabilities, Current	159,367		105,461
Total current liabilities	1,084,059		1,061,946
Operating Lease, Liability, Noncurrent	179,830		0
Long-term Debt, Excluding Current Maturities	1,257,067		1,167,311
Total WESCO International stockholders' equity	2,258,671		2,129,726
Stockholders' Equity Attributable to Parent	2,265,483		2,135,310
Noncontrolling interest	(6,812)		(5,584)	(3,596)	(3,269)
Total liabilities and stockholders' equity	5,017,635		4,605,036
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	1,124		1,123
Total current assets	1,124		1,123
Intercompany receivables, net	0		0
Property, building and equipment, net	0		0
Operating Lease, Right-of-Use Asset	0
Intangible assets, net	0		0
Goodwill and other intangibles, net	0		0
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	3,451,020		3,188,124
Other assets	0		0
Total assets	3,452,144		3,189,247
Accounts payable	0		0
Short-term debt	0		0
Other Liabilities, Current	0		0
Total current liabilities	0		0
Intercompany payables, net	1,186,661		1,053,937
Operating Lease, Liability, Noncurrent	0
Long-term Debt, Excluding Current Maturities	0		0
Liabilities, Other than Long-term Debt, Noncurrent	0		0
Stockholders' Equity Attributable to Parent	2,265,483		2,135,310
Noncontrolling interest	0		0
Total liabilities and stockholders' equity	3,452,144		3,189,247
Guarantor Subsidiaries
Condensed Financial Information
Cash and cash equivalents	46,499		35,931	50,602	41,552
Trade accounts receivable	0		0
Inventories, net	490,071		440,422
Prepaid Expense and Other Assets, Current	44,382		57,586
Total current assets	580,952		533,939
Intercompany receivables, net	0		0
Property, building and equipment, net	82,753		63,506
Operating Lease, Right-of-Use Asset	141,824
Intangible assets, net	1,500		2,131
Goodwill and other intangibles, net	257,623		257,623
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	5,379,728		5,137,783
Other assets	854		2,905
Total assets	6,445,234		5,997,887
Accounts payable	402,174		404,373
Short-term debt	0		0
Other Liabilities, Current	42,901		86,600
Total current liabilities	445,075		490,973
Intercompany payables, net	1,481,653		1,349,767
Operating Lease, Liability, Noncurrent	111,291
Long-term Debt, Excluding Current Maturities	842,708		842,093
Liabilities, Other than Long-term Debt, Noncurrent	113,487		126,930
Stockholders' Equity Attributable to Parent	3,451,020		3,188,124
Noncontrolling interest	0		0
Total liabilities and stockholders' equity	6,445,234		5,997,887
Non-Guarantor Subsidiaries
Condensed Financial Information
Cash and cash equivalents	104,403		60,412	67,351	68,579
Trade accounts receivable	1,187,359		1,166,607
Inventories, net	521,603		508,304
Prepaid Expense and Other Assets, Current	144,084		124,523
Total current assets	1,957,449		1,859,846
Intercompany receivables, net	2,668,314		2,403,704
Property, building and equipment, net	98,695		97,372
Operating Lease, Right-of-Use Asset	94,010
Intangible assets, net	285,775		313,885
Goodwill and other intangibles, net	1,501,417		1,464,980
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0		0
Other assets	12,773		16,994
Total assets	6,618,433		6,156,781
Accounts payable	428,304		389,975
Short-term debt	26,255		30,785
Other Liabilities, Current	183,539		159,481
Total current liabilities	638,098		580,241
Intercompany payables, net	0		0
Operating Lease, Liability, Noncurrent	68,539
Long-term Debt, Excluding Current Maturities	414,359		325,218
Liabilities, Other than Long-term Debt, Noncurrent	124,521		119,123
Stockholders' Equity Attributable to Parent	5,379,728		5,137,783
Noncontrolling interest	(6,812)		(5,584)
Total liabilities and stockholders' equity	6,618,433		6,156,781
Consolidation, Eliminations [Member]
Condensed Financial Information
Operating Lease, Right-of-Use Asset	0
Operating Lease, Liability, Noncurrent	0
Consolidated Entities [Member]
Condensed Financial Information
Cash and cash equivalents	150,902		96,343	117,953	110,131
Trade accounts receivable	1,187,359		1,166,607
Inventories, net	1,011,674		948,726
Prepaid Expense and Other Assets, Current	190,476		173,964
Total current assets	2,540,411		2,385,640
Intercompany receivables, net	0		0
Property, building and equipment, net	181,448		160,878
Operating Lease, Right-of-Use Asset	235,834
Intangible assets, net	287,275		316,016
Goodwill and other intangibles, net	1,759,040		1,722,603
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	0		0
Other assets	13,627		19,899
Total assets	5,017,635		4,605,036
Accounts payable	830,478		794,348
Short-term debt	26,255		30,785
Other Liabilities, Current	227,326		236,813
Total current liabilities	1,084,059		1,061,946
Intercompany payables, net	0		0
Operating Lease, Liability, Noncurrent	179,830
Long-term Debt, Excluding Current Maturities	1,257,067		1,167,311
Liabilities, Other than Long-term Debt, Noncurrent	238,008		246,053
Stockholders' Equity Attributable to Parent	2,265,483		2,135,310
Noncontrolling interest	(6,812)		(5,584)
Total liabilities and stockholders' equity	5,017,635		4,605,036
Consolidation, Eliminations [Member]
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	886		(9,268)
Total current assets	886		(9,268)
Intercompany receivables, net	(2,668,314)		(2,403,704)
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	(8,830,748)		(8,325,907)
Other assets	0		0
Total assets	(11,498,176)		(10,738,879)
Accounts payable	0		0
Short-term debt	0		0
Other Liabilities, Current	886		(9,268)
Total current liabilities	886		(9,268)
Intercompany payables, net	(2,668,314)		(2,403,704)
Long-term Debt, Excluding Current Maturities	0		0
Liabilities, Other than Long-term Debt, Noncurrent	0		0
Stockholders' Equity Attributable to Parent	(8,830,748)		(8,325,907)
Noncontrolling interest	0		0
Total liabilities and stockholders' equity	$ (11,498,176)		$ (10,738,879)