Debt (Committed Funding Facilities) (Details) - Committed funding facilities - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term line of credit	$ 6,665	$ 8,115
Line of credit facility, remaining borrowing capacity	1,935	3,810
Line of credit facility, maximum borrowing capacity	8,600	11,925
Ally Bank | Secured debt
Debt Instrument [Line Items]
Long-term line of credit	3,500	1,785
Line of credit facility, remaining borrowing capacity	1,300	890
Line of credit facility, maximum borrowing capacity	4,800	2,675
Secured funding facilities, revolving, one year or greater
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	7,000
Nonbank funding | Secured debt
Debt Instrument [Line Items]
Long-term line of credit	3,165	6,330
Line of credit facility, remaining borrowing capacity	635	2,920
Line of credit facility, maximum borrowing capacity	$ 3,800	$ 9,250