Debt (Committed Funding Facilities) (Details) - Committed funding facilities - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term line of credit	$ 5,716	$ 6,665
Line of credit facility, remaining borrowing capacity	1,834	1,935
Line of credit facility, maximum borrowing capacity	7,550	8,600
Ally Bank | Secured debt
Debt Instrument [Line Items]
Long-term line of credit	3,050	3,500
Line of credit facility, remaining borrowing capacity	700	1,300
Line of credit facility, maximum borrowing capacity	3,750	4,800
Secured funding facilities, revolving, one year or greater
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	6,000
Nonbank funding | Secured debt
Debt Instrument [Line Items]
Long-term line of credit	2,666	3,165
Line of credit facility, remaining borrowing capacity	1,134	635
Line of credit facility, maximum borrowing capacity	$ 3,800	$ 3,800