| Document and Entity Information Document | Sep. 30, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 30, 2021 |
| Entity Registrant Name | CARMAX, INC. |
| Entity Central Index Key | 0001170010 |
| Amendment Flag | false |
| Entity Incorporation, State or Country Code | VA |
| Entity File Number | 1-31420 |
| Entity Tax Identification Number | 54-1821055 |
| Entity Address, Address Line One | 12800 Tuckahoe Creek Parkway |
| Entity Address, City or Town | Richmond, |
| Entity Address, State or Province | VA |
| Entity Address, Postal Zip Code | 23238 |
| City Area Code | 804 |
| Local Phone Number | 747-0422 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | KMX |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
