Investments (Repurchase Agreements - Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 18,111	$ 19,417
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,861	1,102
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	17,044	18,391
U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,765	1,005
Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	96	97
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	6,594	6,223
Maturity Less than 30 Days | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,760	1,049
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	6,282	6,031
Maturity Less than 30 Days | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,760	1,005
Maturity Less than 30 Days | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	0	44
Maturity 30 to 180 Days [Member]
Securities Financing Transaction [Line Items]
Total	8,024	9,441
Maturity 30 to 180 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	101	53
Maturity 30 to 180 Days [Member] | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	7,269	8,607
Maturity 30 to 180 Days [Member] | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	5	0
Maturity 30 to 180 Days [Member] | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	$ 96	$ 53