Debt (Committed Funding Facilities) (Details) - Committed Funding Facilities [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Revolving Funding Facilities	$ 9,200
Long-term Line of Credit	6,355	$ 8,115
Line of Credit Facility, Remaining Borrowing Capacity	2,870	3,810
Line of Credit Facility, Maximum Borrowing Capacity	9,225	11,925
Ally Bank [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Line of Credit	2,500	1,785
Line of Credit Facility, Remaining Borrowing Capacity	0	890
Line of Credit Facility, Maximum Borrowing Capacity	2,500	2,675
Revolving secured funding facilities 1 year or greater [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	7,600
Nonbank Funding [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Line of Credit	3,855	6,330
Line of Credit Facility, Remaining Borrowing Capacity	2,870	2,920
Line of Credit Facility, Maximum Borrowing Capacity	$ 6,725	$ 9,250