Description of Business and Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2020
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 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	Basis of Presentation The accompanying unaudited interim Consolidated Financial Statements as of March 31, 2020 and for the three months ended March 31, 2020 and 2019 (the "Consolidated Financial Statements") have been prepared in conformity with accounting principles generally accepted in the United States of America ("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 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.
Reclassification	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 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.
Accounts Receivable	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 unemployment rate or gross domestic product growth.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash Restricted cash represents funds that are restricted to satisfy designated current liabilities.
Advertising	Advertising Advertising costs are generally charged to expense in the period incurred.
Recent Accounting Pronouncements	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. 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. The ASU was effective upon its issuance and will remain in effect for all contract modifications and hedging relationships entered into through December 31, 2022. The Company elected the expedient for cash flow hedges to assert the probability of the hedged forecasted transaction under Topic 815: Derivatives and Hedging, regardless of any expected modification in terms related to reference rate reform. This election did not have an impact to the Company’s Consolidated Financial Statements. As the Company begins modifying contracts impacted by reference rate reform, the other optional expedients will be evaluated for impact on the Consolidated Financial Statements. Measurement of Credit Losses on Financial Instruments On January 1, 2020, the Company adopted and applied ASU 2016-13, Financial Instruments - Credit Losses (Topic 326) using the modified retrospective approach. Topic 326 introduced a forward-looking approach based on expected losses to estimate credit losses on certain types of financial instruments, including trade receivables, which is reflected in the Company’s policies. The adoption of Topic 326, as well as the adjustment to retained earnings for the cumulative effect, was not significant to the Company's Consolidated Financial Statements.

Description of Business and Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2020

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 March 31, 2020 and for the three months ended March 31, 2020 and 2019 (the "Consolidated Financial Statements") have been prepared in conformity with accounting principles generally accepted in the United States of America ("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 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.

Reclassification

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 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.

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 unemployment rate or gross domestic product growth.

Cash, Cash Equivalents and Restricted Cash

Restricted cash represents funds that are restricted to satisfy designated current liabilities.

Advertising

Advertising costs are generally charged to expense in the period incurred.

Recent Accounting Pronouncements

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. 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. The ASU was effective upon its issuance and will remain in effect for all contract modifications and hedging relationships entered into through December 31, 2022. The Company elected the expedient for cash flow hedges to assert the probability of the hedged forecasted transaction under Topic 815: Derivatives and Hedging, regardless of any expected modification in terms related to reference rate reform. This election did not have an impact to the Company’s Consolidated Financial Statements. As the Company begins modifying contracts impacted by reference rate reform, the other optional expedients will be evaluated for impact on the Consolidated Financial Statements.

Measurement of Credit Losses on Financial Instruments

On January 1, 2020, the Company adopted and applied ASU 2016-13, Financial Instruments - Credit Losses (Topic 326) using the modified retrospective approach. Topic 326 introduced a forward-looking approach based on expected losses to estimate credit losses on certain types of financial instruments, including trade receivables, which is reflected in the Company’s policies. The adoption of Topic 326, as well as the adjustment to retained earnings for the cumulative effect, was not significant to the Company's Consolidated Financial Statements.