Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 23, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-35985
Entity Registrant Name	CDW CORP
Entity Central Index Key	0001402057
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-0273989
Entity Address, Address Line One	75 Tri-State International
Entity Address, City or Town	Lincolnshire
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60069
City Area Code	847
Local Phone Number	465-6000
Title of 12(b) Security	Common stock, par value $0.01 per share
Trading Symbol	CDW
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 16,514
Entity Common Stock, Shares Outstanding		140,991,095
Documents Incorporated by Reference	Certain parts of the registrant's definitive proxy statement for its 2021 annual meeting of stockholders to be held on May 20, 2021, which will be filed with the Securities and Exchange Commission on or before April 30, 2021, are incorporated by reference into Part III of this Annual Report on Form 10-K.