Regulatory Capital and Other Regulatory Matters - Schedule of Common Share Repurchases (Details) $ / shares in Units, $ in Millions			1 Months Ended				3 Months Ended						6 Months Ended		108 Months Ended
		Jul. 15, 2025 $ / shares	Dec. 31, 2024 USD ($) $ / shares shares	Nov. 30, 2024 USD ($)	Jun. 30, 2024 USD ($) $ / shares shares	Feb. 28, 2023 USD ($)	Jun. 30, 2025 USD ($) $ / shares shares	Mar. 31, 2025 USD ($) $ / shares shares	Dec. 31, 2024 USD ($) $ / shares shares	Sep. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2024 USD ($) $ / shares shares	Mar. 31, 2024 USD ($) $ / shares shares	Jun. 30, 2025 USD ($) $ / shares shares	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2025 $ / shares shares	Oct. 01, 2025	Oct. 31, 2024	Dec. 31, 2023 shares	Oct. 31, 2023	Jun. 30, 2016 shares
Accelerated Share Repurchases [Line Items]
Minimum capital conservation buffer							0.025						0.025		0.025
Proceeds from issuance of long-term debt													$ 953	$ 549
Stock repurchased during period, value							$ 1	$ 34	$ 7	$ 1	$ 1	$ 29
Stock repurchased during period, number of shares (in share) | shares							27,000	877,000	167,000	27,000	13,000	781,000
Common stock, shares outstanding (in shares) | shares			305,387,550		304,656,000		307,786,642	307,152,000	305,387,550	304,715,000	304,656,000	303,978,000	307,786,642	304,656,000	307,786,642			302,459,000		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	$ 0.30
Common stock, share reduction															36.00%
Cash dividends declared per common share (in dollars per share) | $ / shares	[1]						$ 0.30				$ 0.30		$ 0.60	$ 0.60
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 long-term debt				$ 440	$ 330
Automotive | Credit-Linked Notes | Consumer
Accelerated Share Repurchases [Line Items]
Debt instrument, reference portfolio amount				$ 4,000	$ 3,000						$ 3,000			$ 3,000
Ally Financial Inc
Accelerated Share Repurchases [Line Items]
Minimum capital conservation buffer			0.026				0.026		0.026				0.026		0.026		0.026		0.025
Ally Financial Inc | Forecast
Accelerated Share Repurchases [Line Items]
Minimum capital conservation buffer																0.026
Unsecured debt
Accelerated Share Repurchases [Line Items]
Proceeds from issuance of long-term debt			$ 500			$ 500

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