Investments (Repurchase Agreements - Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Total	$ 26,755	$ 30,197
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	102	50
Maturity Less than 30 Days [Member]
Securities Financing Transaction [Line Items]
Total	9,604	12,998
Maturity Less than 30 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	51	0
Maturity 30 to 90 Days [Member]
Securities Financing Transaction [Line Items]
Total	10,543	7,072
Maturity 30 to 90 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	51	50
Foreign Corporate Debt Securities [Member]
Securities Financing Transaction [Line Items]
Total	58	0
Foreign Corporate Debt Securities [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	22	25
Foreign Corporate Debt Securities [Member] | Maturity Less than 30 Days [Member]
Securities Financing Transaction [Line Items]
Total	58	0
Foreign Corporate Debt Securities [Member] | Maturity Less than 30 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	12	0
Foreign Corporate Debt Securities [Member] | Maturity 30 to 90 Days [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Foreign Corporate Debt Securities [Member] | Maturity 30 to 90 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	10	25
Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	80	25
Other Debt Obligations [Member] | Maturity Less than 30 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	39	0
Other Debt Obligations [Member] | Maturity 30 to 90 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	$ 41	$ 25