Investments (Securities Lending and Repurchase Agreements Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	$ 18,283	$ 18,005
U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	17,182	16,951
Foreign government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	1,022	975
RMBS
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	79	79
Securities Financing Transaction, Fair Value | U.S. government and agency
Securities Financing Transaction [Line Items]
Securities Loaned, Not Subject to Master Netting Arrangement	3,500
Open (1)
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	3,532	2,736
Open (1) | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	3,532	2,736
Open (1) | Foreign government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Open (1) | RMBS
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
1 Month or Less
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	5,283	9,288
1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	5,000	8,995
1 Month or Less | Foreign government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	204	214
1 Month or Less | RMBS
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	79	79
Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	9,468	5,981
Over 1 to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	8,650	5,220
Over 1 to 6 Months | Foreign government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	818	761
Over 1 to 6 Months | RMBS
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Repurchase Agreements
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	1,930	1,067
Repurchase Agreements | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	1,860	1,000
Repurchase Agreements | All other corporate and government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	70	67
Repurchase Agreements | Open (1)
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Repurchase Agreements | Open (1) | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Repurchase Agreements | Open (1) | All other corporate and government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Repurchase Agreements | 1 Month or Less
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	1,860	1,000
Repurchase Agreements | 1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	1,860	1,000
Repurchase Agreements | 1 Month or Less | All other corporate and government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Repurchase Agreements | Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	70	67
Repurchase Agreements | Over 1 to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	0	0
Repurchase Agreements | Over 1 to 6 Months | All other corporate and government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	$ 70	$ 67