Regulatory Capital and Other Regulatory Matters (Common Share Repurchases) (Details)			3 Months Ended								12 Months Ended			90 Months Ended
		Jan. 11, 2024 $ / shares	Dec. 31, 2023 USD ($) $ / shares 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	Dec. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 $ / shares shares	Dec. 31, 2023 $ / shares shares	Jul. 31, 2023	Feb. 28, 2023 USD ($)	Aug. 31, 2022	Jan. 10, 2022 USD ($)	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	$ 0.25	$ 0.30
Stock repurchased during period, value			$ 4,000,000	$ 0	$ 2,000,000	$ 27,000,000	$ 51,000,000	$ 415,000,000	$ 600,000,000	$ 584,000,000
Common stock, share reduction														37.00%
Common stock, shares outstanding (in shares) | shares			302,459,258	301,630,000	301,619,000	300,821,000	299,324,357	300,335,000	312,781,000	327,306,000	302,459,258	299,324,357	337,941,000	302,459,258					484,000,000
Minimum capital conservation buffer			0.025								0.025			0.025
Stock repurchased during period, number of shares (in share) | shares			145,000	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]										$ 1.20	$ 1.20	$ 0.88
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											$ 0	$ 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.