| Document and Entity Information | Jan. 22, 2021 |
| Document And Entity Information [Line Items] | |
| Document Period End Date | Jan. 22, 2021 |
| Amendment Flag | false |
| Entity Central Index Key | 0000040729 |
| Document Type | 8-K |
| 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. |
| Entity Address, Address Line Three | 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 |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock, par value $0.01 per share |
| Trading Symbol | ALLY |
| Security Exchange Name | NYSE |
| M 8.125 Fixed Rate Floating Rate Trust Preferred Securities Series 2 Of Gmac Capital Trust I [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 8.125% Fixed Rate/Floating Rate Trust Preferred Securities, Series 2 of GMAC Capital Trust I |
| Trading Symbol | ALLY PRA |
| Security Exchange Name | NYSE |
