| Document and Entity Information | Oct. 21, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0000040729 |
| Document Type | 8-K |
| Document Period End Date | Oct. 21, 2021 |
| Entity File Number | 1-3754 |
| Entity Registrant Name | ALLY FINANCIAL INC. |
| Entity Incorporation State Country Code | DE |
| Entity Tax Identification Number | 38-0572512 |
| Entity Address, Address Line One | Ally Detroit Center |
| Entity Address, Address Line Two | 500 Woodward Ave. Floor 10 |
| Entity Address, City or Town | Detroit |
| Entity Address, State or Province | MI |
| Entity Address, Postal Zip Code | 48226 |
| City Area Code | (866) |
| Local Phone Number | 710-4623 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | ALLY |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
