ACCOUNTING POLICIES (Policies)	12 Months Ended
ACCOUNTING POLICIES (Policies)	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Consolidation	Basis of Consolidation The consolidated financial statements include the accounts of WESCO International and all of its subsidiaries. All significant intercompany accounts and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with generally accepted accounting principles in the United States of America requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Although these estimates are based on management’s best knowledge of current events and actions WESCO may undertake in the future, actual results may ultimately differ from the estimates.
Revenue Recognition	Revenue Recognition WESCO’s revenue arrangements generally consist of single performance obligations to transfer a promised good or service, or a combination of goods and services. Revenue is recognized when control has transferred to the customer, which is generally when the product has shipped from a WESCO facility or directly from a supplier. For products that ship directly from suppliers to customers, WESCO acts as the principal in the transaction and recognizes revenue on a gross basis. Revenue for integrated supply services is recognized over time based on hours incurred as the transfer of control occurs as the services are being performed. WESCO generally satisfies its performance obligations within a year or less. WESCO generally does not have significant financing terms associated with its contracts; payments are normally received within 60 days. There are generally no significant costs associated with obtaining customer contracts. WESCO generally passes through warranties offered by manufacturers or suppliers to its customers. Sales taxes (and value added taxes in foreign jurisdictions) collected from customers and remitted to governmental authorities are excluded from net sales.
Supplier Volume Rebates	Supplier Volume Rebates WESCO receives volume rebates from certain suppliers based on contractual arrangements with such suppliers. Volume rebates are included within other accounts receivable in the Consolidated Balance Sheets, and represent the estimated amounts due to WESCO based on forecasted purchases and the rebate provisions of the various supplier contracts. The corresponding rebate income is recorded as a reduction to cost of goods sold. Receivables under the supplier rebate program were $73.5 million at December 31, 2018 and $72.7 million at December 31, 2017. In 2018, 2017 and 2016, the supplier volume rebate as a percentage of net sales was 1.3%.
Shipping and Handling Costs and Fees	Shipping and handling costs are recognized in net sales when they are billed to the customer. These costs are recognized as a component of selling, general and administrative expenses when WESCO does not bill the customer. WESCO has elected to recognize shipping and handling costs as a fulfillment cost. Shipping and handling costs recorded as a component of selling, general and administrative expenses totaled $74.1 million, $61.8 million and $57.9 million for the years ended December 31, 2018, 2017 and 2016, respectively.
Cash Equivalents	Cash Equivalents Cash equivalents are defined as highly liquid investments with original maturities of 90 days or less when purchased.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts WESCO maintains allowances for doubtful accounts for estimated losses resulting from the inability of its customers to make required payments. WESCO has a systematic procedure using historical data and reasonable assumptions of collectability made at the local branch level and on a consolidated corporate basis to estimate allowances for doubtful accounts. If the financial condition of WESCO’s customers were to deteriorate, resulting in an impairment of their ability to make payments, additional allowances may be required. The allowance for doubtful accounts was $24.5 million at December 31, 2018 and $21.3 million at December 31, 2017. The total amount recorded as selling, general and administrative expense related to bad debts was $10.9 million, $8.5 million and $5.9 million for 2018, 2017 and 2016, respectively.
Inventories	Inventories Inventories primarily consist of merchandise purchased for resale and are stated at the lower of cost and net realizable value. Cost is determined principally under the average cost method. WESCO makes provisions for obsolete or slow-moving inventories as necessary to reflect reductions in value. WESCO writes down its inventories to net realizable value based on internal factors derived from historical analysis of actual losses. WESCO uses past data to identify items in excess of 36 months supply relative to demand or movement. WESCO then analyzes the ultimate disposition of identified excess inventories as they are sold, returned to supplier, or scrapped. This historical item-by-item analysis allows WESCO to develop an estimate of the likelihood that an item identified as being in excess supply ultimately becomes obsolete. WESCO applies the estimate to inventories currently in excess of 36 months supply, and reduces the carrying value of its inventories by the derived amount. Reserves for excess and obsolete inventories were $27.6 million and $28.6 million at December 31, 2018 and 2017, respectively. The total expense related to excess and obsolete inventories, included in cost of goods sold, was $9.7 million, $8.8 million and $7.3 million for 2018, 2017 and 2016, respectively. WESCO absorbs into the cost of inventories certain overhead expenses such as purchasing, receiving and storage and at December 31, 2018 and 2017, $69.2 million and $70.7 million, respectively, of these costs were included in ending inventories.
Property, Buildings and Equipment	Property, Buildings and Equipment Property, buildings and equipment are recorded at cost. Depreciation expense is determined using the straight-line method over the estimated useful lives of the assets. Leasehold improvements are amortized over either their respective lease terms or their estimated lives, whichever is shorter. Estimated useful lives range from five to forty years for buildings and leasehold improvements and three to ten years for furniture, fixtures and equipment. Capitalized computer software costs are amortized using the straight-line method over the estimated useful life, typically three to five years, and are reported at the lower of unamortized cost or net realizable value. Expenditures for new facilities and improvements that extend the useful life of an asset are capitalized. Ordinary repairs and maintenance are expensed as incurred. When property is retired or otherwise disposed of, the cost and the related accumulated depreciation are removed from the accounts and any resulting gains or losses are recorded and reported as selling, general and administrative expenses. Of WESCO’s $160.9 million net book value of property, buildings and equipment as of December 31, 2018, $88.3 million consists of land, buildings and leasehold improvements that are geographically dispersed among WESCO’s 500 branches and 10 distribution centers, mitigating the risk of impairment. WESCO assesses its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of any such assets may not be fully recoverable. Changes in circumstances include technological advances, changes in the business model, capital structure, economic conditions or operating performance. The evaluation is based upon, among other things, utilization, serviceability and assumptions about the estimated future undiscounted cash flows that these assets are expected to generate. When the sum of the undiscounted cash flows is less than the carrying value of the asset or asset group, an impairment loss is recognized to the extent that carrying value exceeds fair value. Management applies its best judgment when performing these evaluations.
Goodwill and Intangible Assets	Goodwill and Indefinite-Lived Intangible Assets Goodwill and indefinite-lived intangible assets are tested for impairment annually during the fourth quarter using information available at the end of September, or more frequently if triggering events occur, indicating that their carrying value may not be recoverable. WESCO tests for goodwill impairment on a reporting unit level and the evaluation involves comparing the fair value of each reporting unit to its carrying value. The fair values of the reporting units are determined using a combination of a discounted cash flow analysis and market multiples. Assumptions used for these fair value techniques are based on a combination of historical results, current forecasts, market data and recent economic events. WESCO evaluates the recoverability of indefinite-lived intangible assets using the relief-from-royalty method based on projected financial information. At December 31, 2018 and 2017, respectively, goodwill and indefinite-lived trademarks totaled $1.8 billion and $1.9 billion. We performed our annual impairment tests of goodwill and indefinite-lived intangible assets during the fourth quarter. A possible indicator of goodwill impairment is the relationship of a company’s market capitalization to its book value. As of December 31, 2018, our market capitalization exceeded our book value and the fair values of our reporting units exceeded their carrying values. Accordingly, there were no impairment losses identified as a result of our annual test. The determination of fair value involves significant management judgment and management applies its best judgment when assessing the reasonableness of financial projections. Fair values are sensitive to changes in underlying assumptions and factors. As a result, there can be no assurance that the estimates and assumptions made for purposes of the annual goodwill and indefinite-lived intangible impairment tests will prove to be an accurate prediction of future results.
Definite Lived Intangible Assets	Definite Lived Intangible Assets Definite lived intangible assets are amortized over 2 to 20 years. A portion of definite lived intangible assets related to certain customer relationships are amortized using an accelerated method whereas all other definite lived intangible assets subject to amortization use a straight-line method. In either case, the amortization method reflects the pattern in which the economic benefits of the respective assets are consumed or otherwise used. Definite lived intangible assets are tested for impairment if events or circumstances occur indicating that the respective asset might be impaired.
Insurance Programs	Insurance Programs WESCO uses commercial insurance for auto, workers’ compensation, casualty and health claims, and information technology as a risk-reduction strategy to minimize catastrophic losses. The Company’s strategy involves large deductible policies where WESCO must pay all costs up to the deductible amount. WESCO estimates the reserve for these programs based on historical incident rates and costs. The assumptions included in developing this accrual include the period of time between the incurrence and payment of a claim. The total liability related to the insurance programs was $13.1 million and $13.9 million at December 31, 2018 and 2017, respectively.
Income Taxes	Income Taxes WESCO accounts for income taxes under the asset and liability method, which requires the recognition of deferred income taxes for events that have future tax consequences. Under this method, deferred income taxes are recognized (using enacted tax laws and rates) based on the future income tax effects of differences in the carrying amounts of assets and liabilities for financial reporting and tax purposes. The effect of a tax rate change on deferred tax assets and liabilities is recognized in income in the period of change. WESCO recognizes deferred tax assets at amounts that are expected to be realized. To make such determination, management evaluates all positive and negative evidence, including but not limited to, prior, current and future taxable income, tax planning strategies and future reversals of existing temporary differences. A valuation allowance is recognized if it is “more-likely-than-not” that some or all of a deferred tax asset will not be realized. WESCO regularly assesses the realizability of deferred tax assets. WESCO accounts for uncertainty in income taxes using a "more-likely-than-not" recognition threshold. Due to the subjectivity inherent in the evaluation of uncertain tax positions, the tax benefit ultimately recognized may materially differ from the estimate. WESCO recognizes interest and penalties related to uncertain tax benefits as part of interest expense and income tax expense, respectively. The Tax Cuts and Jobs Act of 2017 (the “TCJA”) imposed a one-time tax on the deemed repatriation of undistributed foreign earnings (the "transition tax"). Except for a portion of the previously taxed foreign earnings that have been repatriated, WESCO continues to assert that the remaining undistributed earnings of its foreign subsidiaries, the majority of which were subject to the transition tax, are indefinitely reinvested. WESCO believes it is able to maintain a sufficient level of liquidity for its domestic operations and commitments without repatriating cash held by these foreign subsidiaries. Upon any future repatriation, additional income taxes may be incurred; however, it is not practicable to determine the amount at this time. The provisions of the TCJA also introduced U.S. taxation on certain global intangible low-taxed income ("GILTI"). WESCO has elected to account for GILTI tax as a component of income tax expense. Future adjustments (if any) resulting from additional regulatory guidance regarding the accounting for the income tax effects of TCJA will be recognized as discrete income tax expense or benefit in the period in which guidance is issued.
Foreign Currency	Foreign Currency The local currency is the functional currency for the majority of WESCO’s operations outside the United States. Assets and liabilities of these operations are translated to U.S. dollars at the exchange rate in effect at the end of each period. Income statement accounts are translated at an exchange rate that approximates the average for the period. Translation adjustments arising from the use of differing exchange rates from period to period are included as a component of other comprehensive income (loss) within stockholders’ equity. Gains and losses from foreign currency transactions are included in net income for the period.
Defined Benefit Pension Plan	Defined Benefit Pension Plan Liabilities and expenses for pension benefits are determined using actuarial methodologies and incorporate significant assumptions, including the interest rate used to discount the future estimated cash flows, the expected long-term rate of return on plan assets, and several assumptions relating to the employee workforce (salary increases, retirement age, and mortality).
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company measures the fair value of financial assets and liabilities according to a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value and requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of the fair value hierarchy are as follows: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities that are accessible at the measurement date; Level 2 inputs include inputs other than Level 1 that are observable, either directly or indirectly, and Level 3 inputs are unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1) and the lowest priority to measurements involving significant unobservable inputs (Level 3).
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 (see Note 3 below). 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. 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 provided guidance on eight specific cash flow issues where there was 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 consolidated financial statements and notes presented herein. 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 to 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. Early adoption is permitted. Management adopted this ASU in the fourth quarter of 2018 when the Company performed its annual impairment tests. The adoption of this accounting standard did not have an impact on the consolidated financial statements and notes 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 13 for a description of the impact of this accounting standard on the Consolidated Statements of Income and Comprehensive Income presented herein. The adoption of this guidance did not have an impact on the Company's Consolidated Balance Sheets and the 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 consolidated financial statements presented and notes 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. 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 lease standard at the adoption date. Consequently, an entity’s reporting for the comparative periods will continue to be in accordance with current lease accounting guidance. During 2018, 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 migrated to the software solution. The new standard will be adopted in the first quarter of 2019. The Company expects to use the optional transition method and elect the package of practical expedients permitted under the transition guidance. The Company also expects to elect the practical expedient related to lease and nonlease components. The Company anticipates recording right-of-use assets and lease liabilities of $200 million to $250 million in the Consolidated Balance Sheet as of January 1, 2019, most of which will relate to real estate. The Company does not expect the adoption to have a material impact on the Consolidated Statement of Income and Comprehensive Income (Loss) or Consolidated Statement of Cash Flow. The Company is currently updating business processes and internal controls to meet the standard’s new accounting, reporting and disclosure requirements. 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 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.
Accounting Changes and Error Corrections [Text Block]	Correction of Prior Period Management determined that the deferred tax asset related to stock-based compensation was overstated by $5.7 million due to equity awards that had expired subsequent to the requisite service period in years 2016 and prior. In accordance with Staff Accounting Bulletin (“SAB”) No. 99, Materiality, and SAB No. 108, Considering the Effects of Prior Year Misstatements when Quantifying Misstatements in Current Year Financial Statements, management concluded that this misstatement is not material to the Company's previously issued annual and interim financial statements, and the correction of this item in the year ended December 31, 2018 is not material to the 2018 consolidated financial statements presented herein. The Company recorded an adjustment to decrease deferred income taxes and additional capital in the Consolidated Balance Sheet at December 31, 2018.
Reclassification, Policy [Policy 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 13, resulted in the reclassification of amounts reported in the Consolidated Statements of Income and Comprehensive Income for the years ended December 31, 2017 and 2016. The Consolidated Balance Sheet as of December 31, 2017 and the Consolidated Statements of Cash Flows for the years ended December 31, 2017 and 2016 include certain reclassifications to previously reported amounts to conform to the current period's presentation.