Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 26, 2019
Document and Entity Information [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2019  
Document Transition Report false  
Entity File Number 001-35985  
Entity Central Index Key 0001402057  
Current Fiscal Year End Date --12-31  
Entity Registrant Name CDW CORP  
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 Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Emerging Growth Company false  
Entity Small Business false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   144,714,566
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Amendment Flag false