Other Assets - Schedule of Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	$ (80)	$ (131)
Total intangible assets, gross	80	185
Net intangible assets	0	54
Disposal Group, Held-for-Sale or Disposed of by Sale, Not Discontinued Operations | Ally Credit Card
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets	98
Accumulated amortization	47
Technology
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets	39	117
Accumulated amortization	(39)	(77)
Net carrying value	0	40
Customer lists
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets	41	41
Accumulated amortization	(41)	(41)
Net carrying value	0	0
Purchased credit card relationships
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets	0	25
Accumulated amortization	0	(11)
Net carrying value	0	14
Trademarks
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets	0	2
Accumulated amortization	0	(2)
Net carrying value	$ 0	$ 0