Debt (Committed Funding Facilities) (Details) - Committed Funding Facilities Member - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 18,375	$ 20,415
Outstanding	14,800	20,164
Unused capacity	3,575	251
Bank Funding Member | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	3,600	3,250
Outstanding	3,250	3,250
Unused capacity	350	0
Revolving secured funding facilities [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	17,500
Nonbank Funding [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	13,525	17,165
Outstanding	11,550	16,914
Unused capacity	1,975	251
Nonbank Funding [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,250	0
Outstanding	0	0
Unused capacity	1,250	$ 0
Revolving secured funding facilities 1 year or greater [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 14,100