Regulatory Capital and Other Regulatory Matters (Schedule of Regulatory Capital Amount and Ratios) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
BHC Enhanced Prudential Standards, Minimum	$ 100,000
Minimum capital conservation buffer	2.50%	2.50%
Phase-in of capital impact of Accounting Standards Update 2016-13	25.00%
CECL scaling factor	25.00%
BHC stress capital buffer threshold	$ 100,000
BHC capital requirement, dividend payout ratio	30.00%
Risk weighting for certain items not deducted, transitional	250.00%
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%
Ally Financial Inc
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	9.27%	9.54%
Common equity tier one capital	$ 13,544	$ 13,837
Tier one capital to risk-weighted assets, amount	$ 15,952	$ 16,271
Tier one capital to risk-weighted assets, ratio	10.92%	11.22%
Capital to risk-weighted assets, amount	$ 18,645	$ 18,506
Capital to risk-weighted assets, ratio	12.76%	12.76%
Tier one leverage to adjusted quarterly average assets, amount	$ 15,952	$ 16,271
Tier one leverage to adjusted quarterly average assets, ratio	8.92%	9.08%
Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	12.00%	12.30%
Common equity tier one capital	$ 16,433	$ 16,627
Tier one capital to risk-weighted assets, amount	$ 16,433	$ 16,627
Tier one capital to risk-weighted assets, ratio	12.00%	12.30%
Capital to risk-weighted assets, amount	$ 18,145	$ 17,854
Capital to risk-weighted assets, ratio	13.25%	13.21%
Tier one leverage to adjusted quarterly average assets, amount	$ 16,433	$ 16,627
Tier one leverage to adjusted quarterly average assets, ratio	9.87%	10.01%
Minimum
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	4.50%
Tier one capital to risk-weighted assets, required minimum	6.00%
Capital to risk-weighted assets, required minimum	8.00%
Tier one leverage ratio, minimum	4.00%
Minimum | Ally Financial Inc
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	4.50%
Tier one capital to risk-weighted assets, required minimum	6.00%
Capital to risk-weighted assets, required minimum	8.00%
Tier one leverage ratio, minimum	4.00%
Tier one capital to risk-weighted assets, well-capitalized minimum	6.00%
Capital to risk weighted assets, well-capitalzed minimum	10.00%
Minimum | Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio	4.50%
Tier one capital to risk-weighted assets, required minimum	6.00%
Capital to risk-weighted assets, required minimum	8.00%
Tier one leverage ratio, minimum	4.00%
Common Equity Tier One Capital Ratio Required To Be Well Capitalized	6.50%
Tier one capital to risk-weighted assets, well-capitalized minimum	8.00%
Capital to risk weighted assets, well-capitalzed minimum	10.00%
Tier one leverage to adjusted quarterly average assets, well-capitalized minimum	5.00%
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%
Reduction to Common Equity Tier 1 Capital from CECL	$ 1,200