Other Assets - Schedule of Schedule of Goodwill (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Goodwill [Roll Forward]
Goodwill beginning balance			$ 551	$ 669	$ 669
Goodwill impairment	$ 0	$ 0	(305)	0	(118)
Transfer to assets of operations held-for-sale			(56)
Goodwill ending balance	190		190		551
Operating Segments | Automotive Finance operations
Goodwill [Roll Forward]
Goodwill beginning balance			20	20	20
Goodwill impairment			0		0
Transfer to assets of operations held-for-sale			0
Goodwill ending balance	20		20		20
Operating Segments | Insurance operations
Goodwill [Roll Forward]
Goodwill beginning balance			27	27	27
Goodwill impairment			0		0
Transfer to assets of operations held-for-sale			0
Goodwill ending balance	27		27		27
Corporate and Other
Goodwill [Roll Forward]
Goodwill beginning balance			504	$ 622	622
Goodwill impairment			(305)		(118)
Transfer to assets of operations held-for-sale			(56)
Goodwill ending balance	143		143		504
Ally Invest | Corporate and Other
Goodwill [Roll Forward]
Goodwill beginning balance			143
Goodwill ending balance	$ 143		143		143
Ally Credit Card | Corporate and Other
Goodwill [Roll Forward]
Goodwill beginning balance			$ 361
Goodwill ending balance					$ 361