Textuals (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) numberOfPlans	Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)
Schedule of Employee Benefit Plans [Line Items]
Defined contribution plan, employer discretionary contribution amount				$ 20,600
Defined contribution plan, cost recognized	$ 18,300	$ 22,900		42,900
Assets Held-in-trust	$ 39,600
Defined Benefit Plan, Description	WESCO sponsors a contributory defined benefit plan (the "EECOL Plan") covering substantially all Canadian employees of EECOL. The EECOL Plan provides retirement benefits based on earnings and credited service, and participants contribute 2% of their earnings to the EECOL 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	WESCO also sponsors a Supplemental Executive Retirement Plan (the "EECOL SERP"), which provides additional pension benefits to certain executives of EECOL based on earnings, and credited service. Effective January 1, 2013, the EECOL SERP was closed to new participants and existing participants became 100% vested. EECOL SERP participants continue to contribute 4% of their earnings to the EECOL Plan.
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	2.20%	3.20%	[1]
Defined Benefit Plan, Benefit Obligation	$ 819,339	$ 134,852	[1]	105,515	[1]
DefinedBenefitPlanNumberOfPlansWithProjectedBenefitObligationsInExcessOfFairValueOfPlanAssets | numberOfPlans	13
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation	$ 504,800
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets	365,400
Defined Benefit Plan, Plan Assets, Amount	$ 721,005	103,385	[1]	86,556	[1]
Accumulated Benefit Obligation (ABO) at December 31		104,600
DefinedBenefitPlanNumberOfPlansWithAccumulatedBenefitObligationsInExcessOfFairValueOfPlanAssets | numberOfPlans	13
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	$ 435,600
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets	365,400
Service Cost	$ 10,792	4,602	[1]	5,242
Defined Benefit Plan Assets Actual Gain Loss	13.60%
Defined Benefit Plan, Expected Future Benefit Payment, Year One	$ 38,400
WESCO [Member]
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%
Anixter Employee Savings Plan [Member]
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	5.00%
WESCO Deferred Compensation Plan
Schedule of Employee Benefit Plans [Line Items]
Deferred Compensation Liability, Current	$ 10,100
Deferred Compensation Liability, Classified, Noncurrent	17,300	$ 25,200
AnixterDeferred Compensation Plan
Schedule of Employee Benefit Plans [Line Items]
Deferred Compensation Liability, Current	$ 45,100
Foreign Plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	2.00%	3.20%	[1]
Defined Benefit Plan, Benefit Obligation	$ 486,855	$ 134,852	[1]	105,515	[1]
Defined Benefit Plan, Plan Assets, Amount	365,718	103,385	[1]	86,556	[1]
Accumulated Benefit Obligation (ABO) at December 31	417,600
Service Cost	9,029	$ 4,602	[1]	5,242
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	11,400
Defined Benefit Plan, Expected Future Benefit Payment, Year One	$ 9,873
United States
Schedule of Employee Benefit Plans [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	2.60%	0.00%
Defined Benefit Plan, Benefit Obligation	$ 332,484	$ 0		0
Defined Benefit Plan, Plan Assets, Amount	355,287	0		0
Accumulated Benefit Obligation (ABO) at December 31	328,200
Service Cost	1,763	0		0
Defined Benefit Plan, Expected Future Benefit Payment, Year One	$ 28,527
Minimum | WESCO Distribution Canada Defined Contribution plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Defined contribution plan, employer matching contribution, percent	3.00%
Minimum | Anixter Employee Savings Plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Defined contribution discretionary contribution percentage	2.00%
Maximum | WESCO Distribution Canada Defined Contribution plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Defined contribution plan, employer matching contribution, percent	5.00%
Maximum | Anixter Employee Savings Plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Defined contribution discretionary contribution percentage	2.50%
Pension Plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Other Nonoperating Gains (Losses)	$ (8,200)	$ (1,400)		$ (1,900)
Pension Plan [Member] | Foreign Plan [Member]
Schedule of Employee Benefit Plans [Line Items]
Defined Benefit Plan, Benefit Obligation	486,800
Pension Plan [Member] | United States
Schedule of Employee Benefit Plans [Line Items]
Defined Benefit Plan, Expected Future Benefit Payment, Year One	18,100
WESCO Deferred Compensation Plan
Schedule of Employee Benefit Plans [Line Items]
Deferred Compensation Liability, Current and Noncurrent	$ 27,400

[1] For 2019, the changes in benefit obligations, plan assets and funded status relate to the EECOL Plan and the EECOL SERP