ACCOUNTING POLICIES (Policies)	9 Months Ended
ACCOUNTING POLICIES (Policies)	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited Condensed Consolidated Financial Statements of WESCO have been prepared in accordance with Rule 10-01 of Regulation S-X of the Securities and Exchange Commission (the “SEC”). The unaudited condensed consolidated financial information should be read in conjunction with the audited Consolidated Financial Statements and Notes thereto included in WESCO’s 2017 Annual Report on Form 10-K as filed with the SEC on February 22, 2018. The Condensed Consolidated Balance Sheet at December 31, 2017 was derived from the audited Consolidated Financial Statements as of that date, but does not include all of the disclosures required by accounting principles generally accepted in the United States of America. The unaudited Condensed Consolidated Balance Sheet as of September 30, 2018, the unaudited Condensed Consolidated Statements of Income and Comprehensive Income for the nine months ended September 30, 2018 and 2017, respectively, and the unaudited Condensed Consolidated Statements of Cash Flows for the nine months ended September 30, 2018 and 2017, respectively, in the opinion of management, have been prepared on the same basis as the audited Consolidated Financial Statements and include all adjustments necessary for the fair statement of the results of the interim periods presented herein. All adjustments reflected in the unaudited condensed consolidated financial information are of a normal recurring nature unless indicated. The results for the interim periods presented herein are not necessarily indicative of the results to be expected for the full year.
Reclassifications [Text Block]	Reclassifications Effective January 1, 2018, WESCO adopted Accounting Standards Update (ASU) 2017-07, Compensation—Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. The adoption of this ASU, as described below and in Note 8, resulted in the reclassification of amounts reported in the unaudited Condensed Consolidated Statements of Income and Comprehensive Income for the three and nine months ended September 30, 2017.
Fair Value of Financial Instruments	4. FAIR VALUE OF FINANCIAL INSTRUMENTS The Company’s financial instruments consist of cash and cash equivalents, accounts receivable, accounts payable, bank overdrafts, and outstanding indebtedness. The reported carrying amounts of WESCO's financial instruments approximated their fair values as of September 30, 2018 and December 31, 2017. 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 Company's debt instruments are classified as Level 2 within the fair value hierarchy.
New Accounting Pronouncements, Policy [Policy Text Block]	Recently Adopted Accounting Pronouncements Effective January 1, 2018, WESCO adopted ASU 2014-09, Revenue from Contracts with Customers, and all the related amendments (“Topic 606”) using the modified retrospective approach to all open contracts. There was no impact to WESCO’s previously reported consolidated financial statements and WESCO does not expect the adoption of Topic 606 to have a material impact on its revenue and results of operations on an ongoing basis. WESCO’s significant accounting policies are disclosed in Note 2 of the Notes to Consolidated Financial Statements in the Annual Report on Form 10-K for the year ended December 31, 2017. Changes to the Company’s significant accounting policies as a result of adopting Topic 606 are described in Note 3 below. In August 2016, the Financial Accounting Standards Board (FASB) issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force). This ASU provides guidance on eight specific cash flow issues where there is diversity in practice. The Company adopted this ASU in the first quarter of 2018. The adoption of this guidance did not have an impact on the unaudited condensed consolidated financial information presented herein. In March 2017, the FASB issued ASU 2017-07, Compensation—Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. This ASU requires that an employer disaggregate the service cost from the other components of net benefit cost. The Company adopted this guidance on a retrospective basis in the first quarter of 2018. See Note 8 for a description of the impact of this accounting standard on the unaudited Condensed Consolidated Statements of Income and Comprehensive Income presented herein. The adoption of this guidance did not have an impact on the Company's unaudited Condensed Consolidated Balance Sheets and the unaudited Condensed Consolidated Statements of Cash Flows presented herein. In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting. This ASU clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The Company adopted this ASU in the first quarter of 2018. The adoption of this guidance did not have an impact on the unaudited condensed consolidated financial information presented herein. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases, a comprehensive new standard that amends various aspects of existing accounting guidance for leases, including the recognition of a right-of-use asset and a lease liability in the balance sheet and disclosing key information about leasing arrangements. This ASU is effective for fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. During 2018, the FASB issued additional ASUs that address implementation issues and correct or improve certain aspects of the new accounting guidance for leases, including ASU 2018-10, Codification Improvements to Topic 842, Leases and ASU 2018-11, Leases (Topic 842): Targeted Improvements. These ASUs do not change the core principles in the leasing standard outlined above. The amendments in ASU 2018-11 provide an optional transition method that allows entities to initially apply the new leases standard at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Consequently, an entity’s reporting for the comparative periods will continue to be in accordance with current lease accounting guidance. Management established a cross-functional team to evaluate and implement the new standard. The team selected a third-party software solution to facilitate the accounting and financial reporting requirements of the new lease standard. Lease data elements have been gathered and are currently being migrated to the software solution. The new standard will be adopted in the first quarter of 2019 and the Company expects to use the optional transition method. While the Company has not yet completed its evaluation of the effects of adopting this ASU, right-of-use assets and lease liabilities will be recorded in the Consolidated Balance Sheets as of the effective date and thereafter. In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which introduces new guidance for the accounting for credit losses on certain financial instruments. The amendments in this ASU are effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted. Management does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto. In January 2017, the FASB issued ASU 2017-04, Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment, which eliminates Step 2 of the goodwill impairment test. Under the amendments in this ASU, an entity should perform its annual, or interim, goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. An entity should apply the amendments in this ASU on a prospective basis. This guidance is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Management expects to adopt this ASU in the fourth quarter of 2018 when the Company performs its annual impairment testing. The Company does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which amends the disclosure requirements for recurring and nonrecurring fair value measurements by removing, modifying and adding certain disclosures. The amendments in this ASU are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. Management does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto. In August 2018, the FASB issued ASU 2018-14, Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Disclosure Framework—Changes to the Disclosure Requirements for Defined Benefit Plans, which amends the disclosure requirements for all employers that sponsor defined benefit pension and other post retirement plans by removing and adding certain disclosures. The amendments in this ASU are effective for fiscal years ending after December 15, 2020. Early adoption is permitted. Management does not expect the adoption of this accounting standard to have a material impact on its consolidated financial statements and notes thereto. Other pronouncements issued by the FASB or other authoritative accounting standards groups with future effective dates are either not applicable or are not expected to be significant to WESCO’s financial position, results of operations or cash flows.

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.