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Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Apr. 16, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Subsequent Event [Line Items]
Cash dividends declared per common share		$ 0.19	$ 0.17
Goodwill		$ 393		$ 393
Corporate and Other
Subsequent Event [Line Items]
Goodwill		346		346
Ally Invest \| Corporate and Other
Subsequent Event [Line Items]
Goodwill		$ 193		$ 193
Subsequent event \| Ally Invest \| Minimum \| Corporate and Other
Subsequent Event [Line Items]
Goodwill, Impairment Loss	$ 30
Subsequent event \| Ally Invest \| Maximum \| Corporate and Other
Subsequent Event [Line Items]
Goodwill, Impairment Loss	$ 60
Subsequent event \| Common stock
Subsequent Event [Line Items]
Dividends Payable, Date Declared	Apr. 16, 2020
Cash dividends declared per common share	$ 0.19
Dividends Payable, Date to be Paid	May 15, 2020
Dividends Payable, Date of Record	May 01, 2020