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