FAIR VALUE (Notes)	6 Months Ended
FAIR VALUE (Notes)	Jun. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	10. FAIR VALUE OF FINANCIAL INSTRUMENTS The Company’s financial instruments primarily consist of cash and cash equivalents, accounts receivable, accounts payable, bank overdrafts, outstanding indebtedness, foreign currency forward contracts, and benefit plan assets. Except for benefit plan assets, outstanding indebtedness and foreign currency forward contracts, the carrying value of the Company’s remaining financial instruments approximates fair value. The assets of the Company's various defined benefit plans are primarily comprised of common/collective/pool funds (i.e., mutual funds). These funds are valued at the net asset value (NAV) of shares held in the underlying funds. Investments for which fair value is measured using the NAV per share practical expedient are not classified in the fair value hierarchy. The Company uses a market approach to determine the fair value of its debt instruments, utilizing quoted prices in active markets, interest rates and other relevant information generated by market transactions involving similar instruments. Therefore, the inputs used to measure the fair value of the Company's debt instruments are classified as Level 2 within the fair value hierarchy. The carrying value of WESCO's debt instruments with fixed interest rates was $3,230.4 million and $3,730.1 million as of June 30, 2021 and December 31, 2020, respectively. The estimated fair value of this debt was $3,520.2 million and $4,084.7 million as of June 30, 2021 and December 31, 2020, respectively. The reported carrying values of WESCO's other debt instruments, including those with variable interest rates, approximated their fair values as of June 30, 2021 and December 31, 2020. The Company purchases foreign currency forward contracts to minimize the effect of fluctuating foreign currency-denominated accounts on its earnings. The foreign currency forward contracts are not designated as hedges for accounting purposes. The Company's strategy is to negotiate terms for its derivatives and other financial instruments to be highly effective, such that the change in the value of the derivative offsets the impact of the underlying hedge. Its counterparties to foreign currency forward contracts have investment-grade credit ratings. The Company regularly monitors the creditworthiness of its counterparties to ensure no issues exist that could affect the value of its derivatives. The Company does not hedge 100% of its foreign currency-denominated accounts. In addition, the results of hedging can vary significantly based on various factors, such as the timing of executing foreign currency forward contracts versus the movement of currencies as well as the fluctuations in the account balances throughout each reporting period. The fair value of foreign currency forward contracts is based on the difference between the contract rate and the current exchange rate. The fair value of foreign currency forward contracts is measured using observable market information. These inputs would be considered Level 2 in the fair value hierarchy. At June 30, 2021, foreign currency forward contracts were revalued at then-current foreign exchange rates with the changes in valuation reflected directly in other non-operating expenses ("other income, net") in the Condensed Consolidated Statements of Income and Comprehensive Income (Loss) offsetting the transaction gain (loss) recorded on foreign currency-denominated accounts. At June 30, 2021 and December 31, 2020, the gross and net notional amounts of foreign currency forward contracts outstanding were approximately $215.2 million and $111.9 million, respectively. While all of the Company's foreign currency forward contracts are subject to master netting arrangements with its counterparties, assets and liabilities related to these contracts are presented on a gross basis within the Condensed Consolidated Balance Sheets. The gross fair value of assets and liabilities related to foreign currency forward contracts were immaterial.
Derivatives and Fair Value	The Company purchases foreign currency forward contracts to minimize the effect of fluctuating foreign currency-denominated accounts on its earnings. The foreign currency forward contracts are not designated as hedges for accounting purposes. The Company's strategy is to negotiate terms for its derivatives and other financial instruments to be highly effective, such that the change in the value of the derivative offsets the impact of the underlying hedge. Its counterparties to foreign currency forward contracts have investment-grade credit ratings. The Company regularly monitors the creditworthiness of its counterparties to ensure no issues exist that could affect the value of its derivatives. The Company does not hedge 100% of its foreign currency-denominated accounts. In addition, the results of hedging can vary significantly based on various factors, such as the timing of executing foreign currency forward contracts versus the movement of currencies as well as the fluctuations in the account balances throughout each reporting period. The fair value of foreign currency forward contracts is based on the difference between the contract rate and the current exchange rate. The fair value of foreign currency forward contracts is measured using observable market information. These inputs would be considered Level 2 in the fair value hierarchy. At June 30, 2021, foreign currency forward contracts were revalued at then-current foreign exchange rates with the changes in valuation reflected directly in other non-operating expenses ("other income, net") in the Condensed Consolidated Statements of Income and Comprehensive Income (Loss) offsetting the transaction gain (loss) recorded on foreign currency-denominated accounts. At June 30, 2021 and December 31, 2020, the gross and net notional amounts of foreign currency forward contracts outstanding were approximately $215.2 million and $111.9 million, respectively. While all of the Company's foreign currency forward contracts are subject to master netting arrangements with its counterparties, assets and liabilities related to these contracts are presented on a gross basis within the Condensed Consolidated Balance Sheets. The gross fair value of assets and liabilities related to foreign currency forward contracts were immaterial.

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.