| Document and Entity Information | Feb. 16, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 16, 2021 |
| Entity Registrant Name | IAA, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38580 |
| Entity Tax Identification Number | 83-1030538 |
| Entity Address, Address Line One | Two Westbrook Corporate Center |
| Entity Address, Address Line Two | Suite 500 |
| Entity Address, City or Town | Westchester |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60154 |
| City Area Code | 708 |
| Local Phone Number | 492-7000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001745041 |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | IAA |
| Security Exchange Name | NYSE |
