Debt (Committed Funding Facilities) (Details) - Committed Funding Facilities Member - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 16,425	$ 18,375
Long-term Line of Credit	15,423	14,800
Unused capacity	1,002	3,575
Bank Funding Member | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	2,400	3,600
Long-term Line of Credit	2,050	3,250
Unused capacity	350	350
Revolving secured funding facilities [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	15,600
Revolving secured funding facilities 1 year or greater [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	3,100
Nonbank Funding Member | Secured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	12,775	13,525
Long-term Line of Credit	12,123	11,550
Unused capacity	652	1,975
Nonbank Funding Member | Unsecured Debt [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,250	1,250
Long-term Line of Credit	1,250	0
Unused capacity	$ 0	$ 1,250