Regulatory Capital and Other Regulatory Matters (Schedule of Regulatory Capital Amount and Ratios) (Details)	Oct. 01, 2020	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
BHC Enhanced Prudential Standards, Minimum		$ 100,000,000,000
Minimum capital conservation buffer	0.035	0.025	0.025
BHC capital requirement, dividend payout ratio		30.00%
Capital conservation buffer		0.0586	0.0476
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		$ 1,200,000,000
Ally Financial Inc
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio		0.1036	0.0954
Common equity tier one capital		$ 14,258,000,000	$ 13,837,000,000
Tier one capital to risk-weighted assets, amount		$ 16,668,000,000	$ 16,271,000,000
Tier one capital to risk-weighted assets, ratio		0.1211	0.1122
Capital to risk-weighted assets, amount		$ 19,337,000,000	$ 18,506,000,000
Capital to risk-weighted assets, ratio		0.1405	0.1276
Tier one leverage to adjusted quarterly average assets, amount		$ 16,668,000,000	$ 16,271,000,000
Tier one leverage to adjusted quarterly average assets, ratio		0.0900	0.0908
Ally Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one capital ratio		0.1363	0.1230
Minimum capital conservation buffer		2.5
Common equity tier one capital		$ 17,585,000,000	$ 16,627,000,000
Tier one capital to risk-weighted assets, amount		$ 17,585,000,000	$ 16,627,000,000
Tier one capital to risk-weighted assets, ratio		0.1363	0.1230
Capital to risk-weighted assets, amount		$ 19,201,000,000	$ 17,854,000,000
Capital to risk-weighted assets, ratio		0.1488	0.1321
Tier one leverage to adjusted quarterly average assets, amount		$ 17,585,000,000	$ 16,627,000,000
Tier one leverage to adjusted quarterly average assets, ratio		0.1007	0.1001
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