Regulatory Capital and Other Regulatory Matters - Schedule of Common Share Repurchases (Details)			3 Months Ended							9 Months Ended		99 Months Ended
		Oct. 07, 2024 $ / shares	Sep. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2024 USD ($) $ / shares shares	Mar. 31, 2024 USD ($) $ / shares 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	Sep. 30, 2024 $ / shares shares	Sep. 30, 2023 $ / shares shares	Sep. 30, 2024 $ / shares shares	Oct. 31, 2024	Oct. 31, 2023	Feb. 28, 2023 USD ($)	Dec. 31, 2022 shares	Oct. 31, 2022	Jun. 30, 2016 shares
Accelerated Share Repurchases [Line Items]
Minimum capital conservation buffer			0.025							0.025		0.025
Stock repurchased during period, value			$ 1,000,000	$ 1,000,000	$ 29,000,000	$ 4,000,000	$ 0	$ 2,000,000	$ 27,000,000
Stock repurchased during period, number of shares (in share) | shares			27,000	13,000	781,000	145,000	5,000	58,000	836,000
Common stock, shares outstanding (in shares) | shares			304,714,784	304,656,000	303,978,000	302,459,258	301,630,000	301,619,000	300,821,000	304,714,784	301,630,000	304,714,784				299,324,000		484,000,000
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
Common stock, share reduction												37.00%
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
Credit-Linked Notes
Accelerated Share Repurchases [Line Items]
Proceeds from issuance of subordinated long-term debt				$ 330,000,000
Automotive | Credit-Linked Notes | Consumer
Accelerated Share Repurchases [Line Items]
Debt instrument, reference portfolio amount				$ 3,000,000,000
Ally Financial Inc
Accelerated Share Repurchases [Line Items]
Minimum capital conservation buffer			0.025			0.025				0.025		0.025		0.025			0.025
Ally Financial Inc | Subsequent event
Accelerated Share Repurchases [Line Items]
Minimum capital conservation buffer													0.026
Unsecured debt
Accelerated Share Repurchases [Line Items]
Proceeds from issuance of subordinated long-term debt															$ 500,000,000

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