| Document and Entity Information | Jan. 22, 2025 |
| Document And Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 22, 2025 |
| 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 1 | Ally Detroit Center |
| Entity Address Address Line 2 | 500 Woodward Ave. |
| Entity Address Address Line 3 | 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 |
| Entity Emerging Growth Company | false |
| Security Exchange Name | NONE |
| Amendment Flag | false |
| Entity Central Index Key | 0000040729 |
