Other Assets (Schedule of Goodwill) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill	$ 393
Goodwill impairment	(50)	$ 0	$ 0
Goodwill	343	393
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	346
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	$ 193