Regulatory Capital and Other Regulatory Matters (Schedule of Regulatory Capital Amount and Ratios) (Details) - USD ($) $ in Millions	Oct. 01, 2020	Jun. 30, 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	3.50%	2.50%	2.50%
BHC stress capital buffer threshold		$ 100,000
BHC capital requirement, dividend payout ratio		30.00%
Capital conservation buffer		5.59%	4.76%
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%
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		10.09%	9.54%
Common equity tier one capital		$ 13,826	$ 13,837
Tier one capital to risk-weighted assets, amount		$ 16,235	$ 16,271
Tier one capital to risk-weighted assets, ratio		11.85%	11.22%
Capital to risk-weighted assets, amount		$ 18,895	$ 18,506
Capital to risk-weighted assets, ratio		13.79%	12.76%
Tier one leverage to adjusted quarterly average assets, amount		$ 16,235	$ 16,271
Tier one leverage to adjusted quarterly average assets, ratio		8.93%	9.08%
Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio		13.21%	12.30%
Minimum capital conservation buffer		2.50%
Common equity tier one capital		$ 16,918	$ 16,627
Tier one capital to risk-weighted assets, amount		$ 16,918	$ 16,627
Tier one capital to risk-weighted assets, ratio		13.21%	12.30%
Capital to risk-weighted assets, amount		$ 18,522	$ 17,854
Capital to risk-weighted assets, ratio		14.46%	13.21%
Tier one leverage to adjusted quarterly average assets, amount		$ 16,918	$ 16,627
Tier one leverage to adjusted quarterly average assets, ratio		9.94%	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%