Other Assets (Schedule of Goodwill) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
	Apr. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Goodwill [Roll Forward]
Goodwill beginning balance				$ 343
Impairment losses		$ 0	$ 0	0	$ (50)
Goodwill ending balance		343		343
Operating Segments | Automotive Finance operations
Goodwill [Roll Forward]
Goodwill beginning balance				20
Impairment losses				0
Goodwill ending balance		20		20
Operating Segments | Insurance operations
Goodwill [Roll Forward]
Goodwill beginning balance				27
Impairment losses				0
Goodwill ending balance		27		27
Corporate and Other
Goodwill [Roll Forward]
Goodwill beginning balance				296
Impairment losses	$ (50)			0
Goodwill ending balance		296		296
Ally Lending | Corporate and Other
Goodwill [Roll Forward]
Goodwill beginning balance				153
Goodwill ending balance		153		153
Ally Invest | Corporate and Other
Goodwill [Roll Forward]
Goodwill beginning balance				143
Goodwill ending balance		$ 143		$ 143