Regulatory Capital and Other Regulatory Matters (Common Share Repurchases) (Details)			3 Months Ended							9 Months Ended		12 Months Ended	87 Months Ended
		Oct. 09, 2023 $ / shares	Sep. 30, 2023 USD ($) $ / shares shares	Jun. 30, 2023 USD ($) $ / shares shares	Mar. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2022 USD ($) $ / shares shares	Jun. 30, 2022 USD ($) $ / shares shares	Mar. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2023 $ / shares shares	Sep. 30, 2022 $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2023 $ / shares shares	Jul. 31, 2023	Feb. 28, 2023 USD ($)	Aug. 31, 2022	Jan. 10, 2022 USD ($)	Dec. 31, 2021 $ / shares shares	Jun. 30, 2016 shares
Accelerated Share Repurchases [Line Items]
Dividends declared (in dollars per share) | $ / shares			$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30					$ 0.25
Stock repurchased during period, value			$ 0	$ 2,000,000	$ 27,000,000	$ 51,000,000	$ 415,000,000	$ 600,000,000	$ 584,000,000
Common stock, share reduction													38.00%
Common stock, shares outstanding (in shares) | shares			301,629,751	301,619,000	300,821,000	299,324,357	300,335,000	312,781,000	327,306,000	301,629,751	300,335,000	299,324,357	301,629,751					337,941,000	484,000,000
Minimum capital conservation buffer			0.025							0.025			0.025
Stock repurchased during period, number of shares (in share) | shares			5,000	58,000	836,000	1,731,000	12,468,000	15,031,000	12,548,000
Cash dividends declared per common share (in dollars per share) | $ / shares	[1]		$ 0.30				$ 0.30			$ 0.90	$ 0.90
Subsequent event
Accelerated Share Repurchases [Line Items]
Cash dividends declared per common share (in dollars per share) | $ / shares		$ 0.30
Ally Financial Inc
Accelerated Share Repurchases [Line Items]
Minimum capital conservation buffer			0.025	0.025		0.025				0.025		0.025	0.025	0.025		0.025
Unsecured debt
Accelerated Share Repurchases [Line Items]
Proceeds from issuance of subordinated long-term debt															$ 500,000,000
Common stock
Accelerated Share Repurchases [Line Items]
Stock repurchase program, authorized amount																	$ 2,000,000,000
Stock repurchased during period, value												$ 1,650,000,000

[1] Figures in the table may not recalculate exactly due to rounding. Earnings per share is calculated based on unrounded numbers.