Investments (Repurchase Agreements - Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 19,996	$ 20,114
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,062	102
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	19,008	19,359
U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,965	5
Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	97	97
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	8,459	7,108
Maturity Less than 30 Days | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,960	51
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	7,952	6,496
Maturity Less than 30 Days | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,960	5
Maturity Less than 30 Days | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	0	46
Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	7,175	8,526
Maturity 30 to 180 Days | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	102	51
Maturity 30 to 180 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	6,694	8,383
Maturity 30 to 180 Days | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	5	0
Maturity 30 to 180 Days | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	$ 97	$ 51