Regulatory Capital and Other Regulatory Matters (Schedule of Regulatory Capital Amount and Ratios) (Details) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
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
BHC capital requirement, dividend payout ratio	30.00%
Phase-in of capital impact of Accounting Standards Update 2016-13	25.00%
CECL scaling factor	25.00%
Common Equity Tier One Capital – threshold for deduction of certain items, individually	10.00%
Common Equity Tier One Capital - threshold for deduction of certain items, collectively	15.00%
Risk weighting for certain items not deducted	100.00%
Common Equity Tier One Capital - threshold for deduction of certain items, transitional	25.00%
Risk weighting for certain items not deducted, transitional	250.00%
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	0.1064	0.0954
Minimum capital conservation buffer	0.035
Common equity tier one capital	$ 14,878	$ 13,837
Tier one capital to risk-weighted assets, amount	$ 17,289	$ 16,271
Tier one capital to risk-weighted assets, ratio	0.1237	0.1122
Capital to risk-weighted assets, amount	$ 19,778	$ 18,506
Capital to risk-weighted assets, ratio	0.1415	0.1276
Tier one leverage to adjusted quarterly average assets, amount	$ 17,289	$ 16,271
Tier one leverage to adjusted quarterly average assets, ratio	0.0941	0.0908
Capital conservation buffer		0.025
Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	0.1338	0.1230
Minimum capital conservation buffer	0.025
Common equity tier one capital	$ 17,567	$ 16,627
Tier one capital to risk-weighted assets, amount	$ 17,567	$ 16,627
Tier one capital to risk-weighted assets, ratio	0.1338	0.1230
Capital to risk-weighted assets, amount	$ 19,210	$ 17,854
Capital to risk-weighted assets, ratio	0.1463	0.1321
Tier one leverage to adjusted quarterly average assets, amount	$ 17,567	$ 16,627
Tier one leverage to adjusted quarterly average assets, ratio	0.1012	0.1001
Capital conservation buffer	0.025	0.025
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 leverage ratio, minimum	0.0400
Tier one capital to risk-weighted assets, well-capitalized minimum	0.0600
Capital to risk weighted assets, well-capitalzed minimum	0.1000
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
Tier one leverage ratio, minimum	0.0400
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 to adjusted quarterly average assets, well-capitalized minimum	0.0500