Debt (Committed Funding Facilities) (Details) - Committed Funding Facilities Member - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Revolving Funding Facilities	$ 8,700
Line of Credit Facility, Maximum Borrowing Capacity	8,725	$ 11,925
Long-term Line of Credit	5,755	8,115
Line of Credit Facility, Remaining Borrowing Capacity	2,970	3,810
Ally Bank [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,500	2,675
Long-term Line of Credit	1,500	1,785
Line of Credit Facility, Remaining Borrowing Capacity	0	890
Revolving secured funding facilities 1 year or greater [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	6,600
Nonbank Funding [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	7,225	9,250
Long-term Line of Credit	4,255	6,330
Line of Credit Facility, Remaining Borrowing Capacity	$ 2,970	$ 2,920