Description of Business and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Accounting Policies [Abstract]
Description of Business	Description of Business CDW Corporation ("Parent"), a Fortune 500 company and member of the S&P 500 Index, is a leading multi-brand provider of information technology ("IT") solutions to small, medium and large business, government, education and healthcare customers in the United States ("US"), the United Kingdom ("UK") and Canada. The Company’s broad array of offerings ranges from discrete hardware and software products to integrated IT solutions and services that include on-premise, hybrid and cloud capabilities across data center and networking, digital workspace, security and virtualization. Throughout this report, the terms the "Company" and "CDW" refer to Parent and its 100% owned subsidiaries. Parent has two 100% owned subsidiaries, CDW LLC and CDW Finance Corporation. CDW LLC is an Illinois limited liability company that, together with its 100% owned subsidiaries, holds all material assets and conducts all business activities and operations of the Company. CDW Finance Corporation is a Delaware corporation formed for the sole purpose of acting as co-issuer of certain debt obligations and does not hold any material assets or engage in any business activities or operations.
Basis of Presentation	Basis of Presentation The accompanying unaudited interim Consolidated Financial Statements as of September 30, 2021 and for the three and nine months ended September 30, 2021 and 2020 (the "Consolidated Financial Statements") have been prepared in conformity with accounting principles generally accepted in the United States of America ("US GAAP") and the rules and regulations of the US Securities and Exchange Commission (the "SEC") for interim financial statements. Accordingly, certain information and footnote disclosures normally included in financial statements prepared in accordance with US GAAP have been condensed or omitted pursuant to the rules and regulations of the SEC. These Consolidated Financial Statements should be read in conjunction with the audited Consolidated Financial Statements and the notes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2020 (the "December 31, 2020 Consolidated Financial Statements"). The significant accounting policies used in preparing these Consolidated Financial Statements were applied on a basis consistent with those reflected in the December 31, 2020 Consolidated Financial Statements. In the opinion of management, the Consolidated Financial Statements contain all adjustments (consisting of a normal, recurring nature) necessary to present fairly the Company's financial position, results of operations, comprehensive income, cash flows and changes in stockholders' equity as of the dates and for the periods indicated. The unaudited results of operations for such interim periods reported are not necessarily indicative of results for the full year.
Reclassifications	Reclassifications Certain prior period amounts have been reclassified to conform with current period presentation.
Principles of Consolidation	Principles of Consolidation The Consolidated Financial Statements include the accounts of Parent and its 100% owned subsidiaries. All intercompany transactions and accounts are eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of the Consolidated Financial Statements in accordance with US GAAP requires management to make certain estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the Consolidated Financial Statements and the reported amounts of revenue and expenses during the reported periods. The Company bases its estimates on historical experience and on various other assumptions that management believes are reasonable under the circumstances, the results of which form the basis for making judgments about carrying values of assets and liabilities that are not readily apparent from other sources. Actual results and outcomes could differ from those estimates.
Recent Accounting Pronouncements	Accounting for Income Taxes On January 1, 2021, the Company adopted and applied ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes ("Topic 740") in accordance with the applicable transition methods. Among the various updates, the Company adopted the accounting for ownership changes when transitioning from the equity method to consolidation on a modified retrospective basis, which resulted in a $19 million adjustment to retained earnings as of January 1, 2021 for the cumulative effect of derecognizing the deferred tax liability related to the UK acquisition. The adoption of the remaining components of Topic 740 did not have an impact to the Company’s Consolidated Financial Statements. Reference Rate Reform In March 2020, the Financial Accounting Standards Board issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting, followed by an amendment issued in January 2021. This ASU temporarily provides optional expedients and exceptions for applying existing guidance to contract modifications, hedging relationships and other transactions that are expected to be affected by reference rate reform. This ASU, as amended, was effective upon issuance and will remain in effect for all contract modifications and hedging relationships entered into through December 31, 2022. The adoption of this ASU along with the related expedients did not have an impact to the Company’s Consolidated Financial Statements.