Debt (Committed Funding Facilities) (Details) - Committed funding facilities - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Funding facilities, revolving	$ 9,200
Long-term line of credit	6,845	$ 8,115
Line of credit facility, remaining borrowing capacity	2,380	3,810
Line of credit facility, maximum borrowing capacity	9,225	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	0	890
Line of credit facility, maximum borrowing capacity	3,500	2,675
Secured funding facilities, revolving, one year or greater
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	5,000
Nonbank funding | Secured debt
Debt Instrument [Line Items]
Long-term line of credit	3,345	6,330
Line of credit facility, remaining borrowing capacity	2,380	2,920
Line of credit facility, maximum borrowing capacity	$ 5,725	$ 9,250