Other Assets Other Assets (Schedule of Goodwill) (Details) $ in Millions	Mar. 31, 2020 USD ($)
Goodwill [Roll Forward]
Goodwill	$ 393
Goodwill	393
Operating Segments | Automotive Finance operations
Goodwill [Roll Forward]
Goodwill	20
Goodwill	20
Operating Segments | Insurance operations
Goodwill [Roll Forward]
Goodwill	27
Goodwill	27
Corporate and Other
Goodwill [Roll Forward]
Goodwill	346
Goodwill	346
Ally Lending | Corporate and Other
Goodwill [Roll Forward]
Goodwill	153
Goodwill	153
Ally Invest | Corporate and Other
Goodwill [Roll Forward]
Goodwill	193
Goodwill	$ 193