Debt (Committed Funding Facilities) (Details) - Committed Funding Facilities Member - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Outstanding Revolving Funding Facilities	$ 11,900
Line of Credit Facility, Maximum Borrowing Capacity	11,925	$ 18,375
Long-term Line of Credit	8,115	14,800
Line of Credit Facility, Remaining Borrowing Capacity	3,810	3,575
Bank Funding Member | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	2,675	3,600
Long-term Line of Credit	1,785	3,250
Line of Credit Facility, Remaining Borrowing Capacity	890	350
Revolving secured funding facilities 1 year or greater [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	2,600
Nonbank Funding Member | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	9,250	13,525
Long-term Line of Credit	6,330	11,550
Line of Credit Facility, Remaining Borrowing Capacity	2,920	1,975
Nonbank Funding Member | Unsecured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	0	1,250
Long-term Line of Credit	0	0
Line of Credit Facility, Remaining Borrowing Capacity	$ 0	$ 1,250