Regulatory Capital and Other Regulatory Matters (Schedule of Regulatory Capital Amount and Ratios) (Details) $ in Millions	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
BHC Enhanced Prudential Standards, Minimum	$ 100,000
BHC Enhanced Prudential Standards, Maximum	250,000
Average wSTWF exemption threshold	$ 50,000
Minimum capital conservation buffer	0.025
Accounting Standards Update 2016-13
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Phase-in of capital impact of Accounting Standards Update 2016-13	25.00%
CECL scaling factor	25.00%
Deferred reduction to Common Equity Tier 1 Capital from CECL	$ 1,200
Ally Financial Inc
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	0.1106	0.1064
Minimum capital conservation buffer	0.035	0.035
Common equity tier one capital	$ 15,359	$ 14,878
Tier one capital to risk-weighted assets, amount	$ 17,795	$ 17,289
Tier one capital to risk-weighted assets, ratio	0.1282	0.1237
Capital to risk-weighted assets, amount	$ 20,240	$ 19,778
Capital to risk-weighted assets, ratio	0.1458	0.1415
Tier one leverage to adjusted quarterly average assets, amount	$ 17,795	$ 17,289
Tier one leverage to adjusted quarterly average assets, ratio	0.0978	0.0941
Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	0.1368	0.1338
Minimum capital conservation buffer	0.025	0.025
Common equity tier one capital	$ 17,879	$ 17,567
Tier one capital to risk-weighted assets, amount	$ 17,879	$ 17,567
Tier one capital to risk-weighted assets, ratio	0.1368	0.1338
Capital to risk-weighted assets, amount	$ 19,513	$ 19,210
Capital to risk-weighted assets, ratio	0.1493	0.1463
Tier one leverage to adjusted quarterly average assets, amount	$ 17,879	$ 17,567
Tier one leverage to adjusted quarterly average assets, ratio	0.1038	0.1012
Minimum
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	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 | Ally Financial Inc
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	0.0450
Tier one capital to risk-weighted assets, required minimum	0.0600
Capital to risk-weighted assets, required minimum	0.0800
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0600
Capital to risk weighted assets, well-capitalzed minimum	0.1000
Tier one leverage ratio, minimum	0.0400
Minimum | Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	0.0450
Tier one capital to risk-weighted assets, required minimum	0.0600
Capital to risk-weighted assets, required minimum	0.0800
Banking Regulation, Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum	0.0650
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0800
Capital to risk weighted assets, well-capitalzed minimum	0.1000
Tier one leverage ratio, minimum	0.0400
Tier one leverage to adjusted quarterly average assets, well-capitalized minimum	0.0500