| Cover Document | Feb. 06, 2020 |
| Cover Page Document [Abstract] | |
| Entity Central Index Key | 0001402057 |
| Title of 12(b) Security | Common stock, par value $0.01 per share |
| Written Communications | false |
| Entity Incorporation, State or Country Code | DE |
| Document Type | 8-K |
| Document Period End Date | Feb. 6, 2020 |
| Entity Registrant Name | CDW CORP |
| Entity File Number | 001-35985 |
| 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 |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Trading Symbol | CDW |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
