Regulatory Capital and Other Regulatory Matters - Schedule of Regulatory Capital Amount and Ratios (Details) $ in Millions	Oct. 31, 2024	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	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		$ 3,400	$ 3,800
Brokered deposits		$ 9,100
Percentage of interest-bearing domestic deposits to deposits, brokered		6.00%
Common equity tier one capital		$ 15,298	$ 15,129
Common equity tier one capital ratio		0.0979	0.0936
Tier one capital to risk-weighted assets, amount		$ 17,564	$ 17,392
Tier one capital to risk-weighted assets, ratio		0.1124	0.1076
Tier one capital to risk-weighted assets, well-capitalized minimum		0.0600
Capital to risk-weighted assets, amount		$ 20,173	$ 20,055
Capital to risk-weighted assets, ratio		0.1290	0.1241
Capital to risk weighted assets, well-capitalized minimum		0.1000
Tier one leverage to adjusted quarterly average assets, amount		$ 17,564	$ 17,392
Tier one leverage to adjusted quarterly average assets, ratio		0.0899	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		0.025	0.025	0.025	0.025
Ally Financial Inc | Subsequent event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum capital conservation buffer	0.026
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
Common equity tier one capital		$ 17,579	$ 17,217
Common equity tier one capital ratio		0.1192	0.1124
Common equity tier one capital, well capitalized minimum		0.0650
Tier one capital to risk-weighted assets, amount		$ 17,579	$ 17,217
Tier one capital to risk-weighted assets, ratio		0.1192	0.1124
Tier one capital to risk-weighted assets, well-capitalized minimum		0.0800
Capital to risk-weighted assets, amount		$ 19,441	$ 19,144
Capital to risk-weighted assets, ratio		0.1318	0.1250
Capital to risk weighted assets, well-capitalized minimum		0.1000
Tier one leverage to adjusted quarterly average assets, amount		$ 17,579	$ 17,217
Tier one leverage to adjusted quarterly average assets, ratio		0.0952	0.0907
Tier one leverage to adjusted quarterly average assets, well-capitalized minimum		0.0500