Investments (Repurchase Agreements - Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 19,417	$ 20,114
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,102	102
Maturity Less than 30 Days [Member]
Securities Financing Transaction [Line Items]
Total	6,223	7,108
Maturity Less than 30 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,049	51
Maturity 30 to 90 Days [Member]
Securities Financing Transaction [Line Items]
Total	9,441	8,526
Maturity 30 to 90 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	53	51
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	18,391	19,359
U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,005	5
U.S. government and agency | Maturity Less than 30 Days [Member]
Securities Financing Transaction [Line Items]
Total	6,031	6,496
U.S. government and agency | Maturity Less than 30 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,005	5
U.S. government and agency | Maturity 30 to 90 Days [Member]
Securities Financing Transaction [Line Items]
Total	8,607	8,383
U.S. government and agency | Maturity 30 to 90 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	97	97
Other Debt Obligations [Member] | Maturity Less than 30 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	44	46
Other Debt Obligations [Member] | Maturity 30 to 90 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	$ 53	$ 51