| Document and Entity Information Document | Nov. 05, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Nov. 5, 2019 |
| Entity Registrant Name | KAR Auction Services, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-34568 |
| Entity Tax Identification Number | 20-8744739 |
| Entity Address, Address Line One | 11299 N. Illinois Street |
| Entity Address, City or Town | Carmel |
| Entity Address, State or Province | IN |
| Entity Address, Postal Zip Code | 46032 |
| City Area Code | 800 |
| Local Phone Number | 923-3725 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | KAR |
| Security Exchange Name | NYSE |
| Entity Central Index Key | 0001395942 |
| Amendment Flag | false |
