Description of Business and Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2017
Accounting Policies [Abstract]
Description of Business	Description of Business CDW Corporation (“Parent”) is a Fortune 500 company with multi-national capabilities and a leading provider of integrated information technology (“IT”) solutions to small, medium and large business, government, education and healthcare customers in the United States, Canada and the United Kingdom. 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 June 30, 2017 and for the three and six months ended June 30, 2017 and 2016 (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 included in the Company's Annual Report on Form 10-K for the year ended December 31, 2016 (the “December 31, 2016 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, 2016 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, 2016 Consolidated Financial Statements include a discussion of the significant accounting policies and estimates used in the preparation of the Company's Consolidated Financial Statements. There have been no material changes to the Company's significant accounting policies and estimates during the six months ended June 30, 2017.
Accounting for Derivative Instruments	Accounting for Derivative Instruments The Company has entered into interest rate cap agreements for the purpose of hedging its exposure to fluctuations in interest rates. The interest rate cap agreements are designated as cash flow hedges of interest rate risk and recorded at fair value in Other assets on the Consolidated Balance Sheets. The gain or loss on the derivative instruments is reported as a component of Accumulated Other Comprehensive Loss (“AOCL”) until reclassified to Interest expense in the same period the hedge transaction affects earnings.
Recent Accounting Pronouncements	Accounting for Goodwill Impairment In January 2017, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standard Update (“ASU”) 2017-04, Simplifying the Test for Goodwill Impairment (Topic 350), providing guidance for the elimination of Step 2 of the current two-step goodwill impairment test. A goodwill impairment loss will be measured as the excess of a reporting unit's carrying amount over its fair value. This ASU 2017-04 is effective for the Company beginning in the first quarter of 2020 and allows for early adoption. The adoption of this ASU is not expected to have a material impact on the Company's Consolidated Financial Statements. Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments (Topic 230), providing guidance for eight specific cash flow issues with the objective of reducing the existing diversity in practice. This ASU 2016-15 is effective for the Company beginning in the first quarter of 2018 and allows for early adoption. The adoption of this ASU is not expected to have a material impact of the Company's Consolidated Financial Statements. Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. This 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. This ASU also expands the disclosure requirements to enable users of financial statements to understand the assumptions, models and methods for estimating expected credit losses. This ASU is effective for the Company beginning in the first quarter of 2020 and allows for early adoption beginning in the first quarter of 2019. The Company is currently evaluating the impact the ASU will have on its Consolidated Financial Statements. Accounting for Leases In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), requiring lessees to recognize assets and liabilities on the balance sheet for the rights and obligations created by long-term leases and to disclose additional quantitative and qualitative information about leasing arrangements. This ASU is effective for the Company beginning in the first quarter of 2019 and allows for early adoption. Although the Company is currently evaluating the provisions of the ASU to determine how it will be affected, the primary impact to the Company of the new ASU will be to record assets and liabilities for current operating leases. The majority of the Company’s current operating leases relate to its real estate portfolio. Revenue Recognition In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which, along with amendments issued in 2015 and 2016, will replace most existing revenue recognition guidance under GAAP and eliminate industry-specific guidance. The core principle of the new guidance is that an entity should recognize revenue for the transfer of goods and services equal to an amount it expects to be entitled to receive for those goods and services. The ASU, as amended, will be effective for the Company beginning in the first quarter of 2018, and allows for early adoption in the first quarter of 2017. The new guidance permits two methods of adoption: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the cumulative catch-up transition method). The Company has established a cross-functional implementation team to analyze the effect of the ASU. The Company utilized a bottom-up approach to analyze the impact of the standard on its contract portfolio by reviewing the current accounting policies and practices to identify potential differences that would result from applying the requirements of the new standard to its revenue contracts. In addition, the Company identified, and is in the process of implementing, appropriate changes to its business processes, systems and controls to support recognition and disclosure under the new standard. The implementation team has reported the findings and progress of the project to management and the Audit Committee on a frequent basis. The Company will adopt the guidance on January 1, 2018, and expects to utilize the full retrospective method. The Company’s ability to adopt using the full retrospective method is dependent on system readiness and the completion of its analysis of information necessary to recast prior period financial statements. While the Company is still finalizing its accounting policies under the new standard, it has determined: •the accounting for bill and hold transactions will result in revenue for certain of those arrangements being recognized earlier than under current GAAP. This change will not materially impact Net sales or Net income; •it will be deemed to be acting as an agent in transactions for certain software products sold with accompanying third-party delivered software assurance and therefore will recognize such revenue on a net basis. The Company currently recognizes these on a gross basis (i.e. acting as a principal). While this change will not impact reported Gross profit, the Company estimates the impact of this on the 2016 full-year results would have been to reduce both Net sales and Cost of sales by $200 - $300 million; and •the accounting for revenue related to hardware, software (excluding the above) and professional services will remain substantially unchanged.