Regulatory Capital and Other Regulatory Matters - Schedule of Regulatory Capital Amount and Ratios (Details) $ in Millions	Dec. 31, 2024 USD ($)	Oct. 31, 2024	Dec. 31, 2023 USD ($)	Oct. 31, 2023	Oct. 31, 2022
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	$ 3,900		$ 3,800
Brokered deposits	$ 6,700
Percentage of interest-bearing domestic deposits to deposits, brokered	4.40%
Common equity tier one capital	$ 15,058		$ 15,129
Common equity tier one capital ratio	0.0982		0.0936
Tier one capital to risk-weighted assets, amount	$ 17,324		$ 17,392
Tier one capital to risk-weighted assets, ratio	0.1130		0.1076
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0600
Capital to risk-weighted assets, amount	$ 20,182		$ 20,055
Capital to risk-weighted assets, ratio	0.1316		0.1241
Capital to risk weighted assets, well-capitalized minimum	0.1000
Tier one leverage to adjusted quarterly average assets, amount	$ 17,324		$ 17,392
Tier one leverage to adjusted quarterly average assets, ratio	0.0892		0.0867
Accounting Standards Update 2016-13
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Deferred reduction to Common Equity Tier 1 Capital from CECL	$ 296
Ally Financial Inc
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum capital conservation buffer	0.026	0.026	0.025	0.025	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	$ 17,229		$ 17,217
Common equity tier one capital ratio	0.1194		0.1124
Common equity tier one capital, well capitalized minimum	0.0650
Tier one capital to risk-weighted assets, amount	$ 17,229		$ 17,217
Tier one capital to risk-weighted assets, ratio	0.1194		0.1124
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0800
Capital to risk-weighted assets, amount	$ 19,052		$ 19,144
Capital to risk-weighted assets, ratio	0.1321		0.1250
Capital to risk weighted assets, well-capitalized minimum	0.1000
Tier one leverage to adjusted quarterly average assets, amount	$ 17,229		$ 17,217
Tier one leverage to adjusted quarterly average assets, ratio	0.0940		0.0907
Tier one leverage to adjusted quarterly average assets, well-capitalized minimum	0.0500