Description of Business and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
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 provider of integrated information technology ("IT") solutions with multi-national capabilities 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 as described in Note 13 (Supplemental Guarantor Information) 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, 2019 and for the three and nine months ended September 30, 2019 and 2018 (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, 2018 (the "December 31, 2018 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, 2018 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.
Principles of Consolidation	Principles of Consolidation The accompanying 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 use of 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 could differ from those estimates. The notes to the Consolidated Financial Statements contained in the December 31, 2018 Consolidated Financial Statements include a discussion of the significant accounting policies and estimates used in the preparation of the Company's Consolidated Financial Statements. Except as noted below for the adoption of Accounting Standards Update ("ASU") 2016-02, Leases ("Topic 842"), there have been no material changes to the Company's significant accounting policies and estimates during the nine months ended September 30, 2019.
Leases	Leases The Company enters into operating lease contracts, as assessed at contract inception, primarily for real estate, data centers and equipment. On the Company's Consolidated Balance Sheets, Operating lease assets are included in Operating lease right-of-use assets and Operating lease liabilities are included in Other current liabilities and Long-term operating lease liabilities. At the lease commencement date, the Company records Operating lease liabilities based on the present value of the future lease payments. In determining the present value of future lease payments, the Company uses its incremental borrowing rate based on the information available at the commencement date. For real estate and data center contracts, the Company accounts for the lease and non-lease components as a single lease component. For certain equipment leases, the Company applies a portfolio approach to account for the right-of-use asset and operating lease liability. In assessing the lease term, the Company includes options to renew only when it is reasonably certain that it will be exercised; a determination which is at the sole discretion of the Company. Generally, for leases with an initial term of 12 months or less, the Company has elected to not record a right-of-use asset and lease liability. For equipment leases used in revenue generating activities, the Company will record a right-of-use asset and lease liability for leases with a term of 12 months or less. The Company records lease expense on a straight-line basis over the lease term beginning on the commencement date.
Recent Accounting Pronouncements	Measurement of Credit Losses on Financial Instruments In June 2016, the Financial Accounting Standards Board issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The ASU introduces a new forward-looking approach, based on expected losses, to estimate credit losses on certain types of financial instruments, including trade receivables. The estimate of expected credit losses will require considerations of historical information, current information and reasonable and supportable forecasts. The ASU also expands the disclosure requirements to enable users of financial statements to understand the assumptions, models and methods for estimating expected credit losses. The ASU will be adopted on January 1, 2020. The Company has established a cross-functional implementation team to analyze the effect of the ASU. The analysis includes identifying pools of receivables, evaluating and assessing estimation methodologies, assessing policy elections, and evaluating its business processes and internal controls to meet the ASU’s accounting, reporting and disclosure requirements. The Company is currently evaluating the impact the ASU will have on its Consolidated Financial Statements. Accounting for Leases On January 1, 2019, the Company adopted and applied Topic 842 resulting in the recognition of right-of-use assets and additional lease liabilities of $81 million as of January 1, 2019, mainly related to operating leases for the Company's real estate portfolio. For additional details regarding the impact to the Company's Consolidated Balance Sheets from adopting this standard, see Note 9 (Leases).