Other Assets (Schedule of Goodwill) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Goodwill [Roll Forward]
Goodwill			$ 393
Goodwill impairment	$ 0	$ 0	(50)	$ 0
Goodwill	343		343
Operating Segments | Automotive Finance operations
Goodwill [Roll Forward]
Goodwill			20
Goodwill impairment			0
Goodwill	20		20
Operating Segments | Insurance operations
Goodwill [Roll Forward]
Goodwill			27
Goodwill impairment			0
Goodwill	27		27
Corporate and Other
Goodwill [Roll Forward]
Goodwill			346
Goodwill impairment			(50)
Goodwill	296		296
Ally Lending | Corporate and Other
Goodwill [Roll Forward]
Goodwill			153
Goodwill	153		153
Ally Invest | Corporate and Other
Goodwill [Roll Forward]
Goodwill			193
Goodwill	$ 143		$ 143