| Document and Entity Information Document | Jan. 12, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jan. 12, 2021 |
| Entity File Number | 1-3754 |
| Entity Registrant Name | ALLY FINANCIAL INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 38-0572512 |
| Entity Address, Address Description | Ally Detroit Center |
| Entity Address, Address Line One | 500 Woodward Ave. |
| Entity Address, Address Line Two | 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 |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000040729 |
| Amendment Flag | false |
| Common Stock | |
| Entity Information [Line Items] | |
| Security Exchange Name | NYSE |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | ALLY |
| Trust Preferred Securities Subject to Mandatory Redemption [Member] | |
| Entity Information [Line Items] | |
| Security Exchange Name | NYSE |
| Title of 12(b) Security | 8.125% Fixed Rate/Floating Rate Trust Preferred Securities, Series 2 of GMAC Capital Trust I |
| Trading Symbol | ALLY PRA |
