Investments (Securities Lending and Repurchase Agreements Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	$ 17,782	$ 18,005
U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	16,668	16,951
Foreign government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	1,046	975
RMBS
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	68	79
Securities Financing Transaction, Fair Value | U.S. government and agency
Securities Financing Transaction [Line Items]
Securities Loaned, Not Subject to Master Netting Arrangement	3,400
Open (1)
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	3,440	2,736
Open (1) | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	3,440	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	4,595	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	4,147	8,995
1 Month or Less | Foreign government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	380	214
1 Month or Less | RMBS
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	68	79
Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	9,747	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	9,081	5,220
Over 1 to 6 Months | Foreign government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	666	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	1,612	1,067
Repurchase Agreements | U.S. government and agency
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	1,535	1,000
Repurchase Agreements | All other corporate and government
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	77	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	1,541	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	1,535	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	6	0
Repurchase Agreements | Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Transfers Accounted for as Secured Borrowings, Associated Liabilities, Carrying Amount	71	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	$ 71	$ 67