Regulatory Capital and Other Regulatory Matters - Schedule of Regulatory Capital Amount and Ratios (Details) $ in Millions	Jun. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)	Oct. 31, 2024	Oct. 31, 2023
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio, minimum	0.045
Tier one capital to risk-weighted assets, required minimum	0.06
Capital to risk-weighted assets, required minimum	0.08
Tier one leverage ratio, minimum	0.04
Minimum capital conservation buffer	0.025
Accumulated other comprehensive losses excluded from Common Equity Tier 1 Capital	$ 3,200	$ 3,900
Brokered deposits	$ 3,200
Percentage of interest-bearing domestic deposits to deposits, brokered	2.20%
Common equity tier one capital	$ 14,960	$ 15,058
Common equity tier one capital ratio	0.0989	0.0982
Tier one capital to risk-weighted assets, amount	$ 17,216	$ 17,324
Tier one capital to risk-weighted assets, ratio	0.1138	0.1130
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0600
Capital to risk-weighted assets, amount	$ 20,041	$ 20,182
Capital to risk-weighted assets, ratio	0.1325	0.1316
Capital to risk weighted assets, well-capitalized minimum	0.1000
Tier one leverage to adjusted quarterly average assets, amount	$ 17,216	$ 17,324
Tier one leverage to adjusted quarterly average assets, ratio	0.0906	0.0892
Ally Financial Inc
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum capital conservation buffer	0.026	0.026	0.026	0.025
Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio, minimum	0.0450
Tier one capital to risk-weighted assets, required minimum	0.0600
Capital to risk-weighted assets, required minimum	0.0800
Tier one leverage ratio, minimum	0.0400
Minimum capital conservation buffer	0.025	0.025
Common equity tier one capital	$ 17,664	$ 17,229
Common equity tier one capital ratio	0.1248	0.1194
Common equity tier one capital, well capitalized minimum	0.0650
Tier one capital to risk-weighted assets, amount	$ 17,664	$ 17,229
Tier one capital to risk-weighted assets, ratio	0.1248	0.1194
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0800
Capital to risk-weighted assets, amount	$ 19,454	$ 19,052
Capital to risk-weighted assets, ratio	0.1374	0.1321
Capital to risk weighted assets, well-capitalized minimum	0.1000
Tier one leverage to adjusted quarterly average assets, amount	$ 17,664	$ 17,229
Tier one leverage to adjusted quarterly average assets, ratio	0.0987	0.0940
Tier one leverage to adjusted quarterly average assets, well-capitalized minimum	0.0500