Description of Business and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Description of Business and Summary of Significant Accounting Policies	Description of Business and Summary of Significant Accounting Policies Description of Business CDW Corporation ("Parent"), a Fortune 500 company and member of the S&P 500 Index, is a market-leading provider of integrated 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 offerings range from discrete hardware and software products to integrated IT solutions such as mobility, security, data center optimization, cloud computing, virtualization and collaboration. 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 The accompanying unaudited interim Consolidated Financial Statements as of September 30, 2020 and for the three and nine months ended September 30, 2020 and 2019 (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, 2019 (the "December 31, 2019 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, 2019 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 Certain prior period amounts have been reclassified to conform with current period presentation. 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 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 including management’s current assumptions with respect to implications of the novel coronavirus (“COVID-19”) pandemic, 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. The notes to the Consolidated Financial Statements contained in the December 31, 2019 Consolidated Financial Statements include a discussion of the Company's significant accounting policies and estimates. Except as noted within Note 2 (Recent Accounting Pronouncements) for the adoption of Accounting Standards Update ("ASU") 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments ("Topic 326"), there have been no changes to the Company's significant accounting policies and estimates during the nine months ended September 30, 2020. Accounts Receivable The Company estimates an allowance for credit losses related to accounts receivable for future expected credit losses by using relevant information such as historical information, current conditions, and reasonable and supportable forecasts. The allowance is measured on a pool basis when similar risk characteristics exist, and a loss-rate for each pool is determined using historical credit loss experience as the basis for the estimation of expected credit losses. Adjustments to historical loss information are made for differences in current conditions as well as changes in forecasted macroeconomic conditions, such as changes in the unemployment rate or gross domestic product growth rate. The Company has typically experienced a higher loss-rate experience with customers in pools associated with the Company's Corporate and Small Business segments, as compared to the pools associated with the Public segment. During the nine months ended September 30, 2020, the Company recognized an allowance of $30 million to reflect the forecasted credit deterioration due to the COVID-19 pandemic, which considered the customer makeup and overall size of the Company's pools, the impacts experienced to date and the impacts from the last significant economic downturn in 2008-2009. Due to the higher inherent risk in the pools associated with the Company's Corporate and Small Business segments, and the overall size of certain pools within the Public segment, the majority of the additional allowance relates to these segments. As the overall impact and duration of the COVID-19 pandemic remains uncertain, the Company's estimates and assumptions may evolve as conditions change. Advertising Advertising costs are generally charged to expense in the period incurred and recorded in Selling and administrative expenses in the Consolidated Statements of Operations. During the three months ended September 30, 2020 and 2019, the Company had advertising costs of $47 million and $51 million, respectively. During the nine months ended September 30, 2020 and 2019, the Company had advertising costs of $140 million and $143 million, respectively.