Investments (Securities Lending and Repurchase Agreements Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 18,005	$ 19,417
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	16,951	18,391
Debt Security, Government, Non-US [Member]
Securities Financing Transaction [Line Items]
Total	975	1,026
RMBS
Securities Financing Transaction [Line Items]
Total	79	0
Open
Securities Financing Transaction [Line Items]
Total	2,736	3,753
Open | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	2,736	3,753
Open | Debt Security, Government, Non-US [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Open | RMBS
Securities Financing Transaction [Line Items]
Total	0	0
1 Month or Less
Securities Financing Transaction [Line Items]
Total	9,288	6,223
1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	8,995	6,031
1 Month or Less | Debt Security, Government, Non-US [Member]
Securities Financing Transaction [Line Items]
Total	214	192
1 Month or Less | RMBS
Securities Financing Transaction [Line Items]
Total	79	0
Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Total	5,981	9,441
Over 1 to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	5,220	8,607
Over 1 to 6 Months | Debt Security, Government, Non-US [Member]
Securities Financing Transaction [Line Items]
Total	761	834
Over 1 to 6 Months | RMBS
Securities Financing Transaction [Line Items]
Total	0	0
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,067	1,102
Repurchase Agreements [Member] | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	1,000	1,005
Repurchase Agreements [Member] | Other Debt Obligations [Member]
Securities Financing Transaction [Line Items]
Total	67	97
Repurchase Agreements [Member] | Open
Securities Financing Transaction [Line Items]
Total	0	0
Repurchase Agreements [Member] | Open | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	0	0
Repurchase Agreements [Member] | Open | Other Debt Obligations [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Repurchase Agreements [Member] | 1 Month or Less
Securities Financing Transaction [Line Items]
Total	1,000	1,049
Repurchase Agreements [Member] | 1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	1,000	1,005
Repurchase Agreements [Member] | 1 Month or Less | Other Debt Obligations [Member]
Securities Financing Transaction [Line Items]
Total	0	44
Repurchase Agreements [Member] | Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Total	67	53
Repurchase Agreements [Member] | Over 1 to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	0	0
Repurchase Agreements [Member] | Over 1 to 6 Months | Other Debt Obligations [Member]
Securities Financing Transaction [Line Items]
Total	$ 67	$ 53