Regulatory Capital and Other Regulatory Matters (Schedule of Regulatory Capital Amount and Ratios) (Details) $ in Millions	Sep. 30, 2023 USD ($)	Jul. 31, 2023	Jun. 30, 2023	Dec. 31, 2022 USD ($)	Aug. 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
Minimum capital conservation buffer	0.025
Accumulated other comprehensive losses excluded from Common Equity Tier 1 Capital	$ 4,800			$ 4,100
Brokered deposits	$ 11,300
Percentage of interest-bearing domestic deposits to deposits, brokered	7.40%
Common equity tier one capital	$ 14,994			$ 14,592
Common equity tier one capital ratio	0.0931			0.0927
Tier one capital to risk-weighted assets, amount	$ 17,257			$ 16,867
Tier one capital to risk-weighted assets, ratio	0.1071			0.1072
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0600
Capital to risk-weighted assets, amount	$ 20,126			$ 19,209
Capital to risk-weighted assets, ratio	0.1249			0.1221
Capital to risk weighted assets, well-capitalized minimum	0.1000
Tier one leverage to adjusted quarterly average assets, amount	$ 17,257			$ 16,867
Tier one leverage to adjusted quarterly average assets, ratio	0.0860			0.0865
Tier one leverage ratio, minimum	0.04
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	$ 591
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	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
Common equity tier one capital	$ 17,215			$ 17,011
Common equity tier one capital ratio	0.1127			0.1138
Common equity tier one capital, well capitalized minimum	0.0650
Tier one capital to risk-weighted assets, amount	$ 17,215			$ 17,011
Tier one capital to risk-weighted assets, ratio	0.1127			0.1138
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0800
Capital to risk-weighted assets, amount	$ 19,140			$ 18,888
Capital to risk-weighted assets, ratio	0.1253			0.1264
Capital to risk weighted assets, well-capitalized minimum	0.1000
Tier one leverage to adjusted quarterly average assets, amount	$ 17,215			$ 17,011
Tier one leverage to adjusted quarterly average assets, ratio	0.0904			0.0923
Tier one leverage ratio, minimum	0.0400
Tier one leverage to adjusted quarterly average assets, well-capitalized minimum	0.0500