Investments (Securities Lending, Repurchase Agreements and FHLB Agreements Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	$ 19,718	$ 17,369
U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	18,606	16,267
Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	1,030	1,026
RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	73	76
U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	9	0
Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	800	800
Open (1)
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	3,974	2,928
Open (1) | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	3,965	2,928
Open (1) | Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Open (1) | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Open (1) | U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	9	0
Open (1) | Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	0	0
1 Month or Less
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	8,457	7,011
1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	8,116	6,676
1 Month or Less | Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	268	259
1 Month or Less | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	73	76
1 Month or Less | U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
1 Month or Less | Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	250	250
Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	7,287	7,430
Over 1 to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	6,525	6,663
Over 1 to 6 Months | Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	762	767
Over 1 to 6 Months | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 1 to 6 Months | U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 1 to 6 Months | Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	550	475
Maturity 180 to 360 Days [Member] [Domain]
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Maturity 180 to 360 Days [Member] [Domain] | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Maturity 180 to 360 Days [Member] [Domain] | Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Maturity 180 to 360 Days [Member] [Domain] | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Maturity 180 to 360 Days [Member] [Domain] | U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Maturity 180 to 360 Days [Member] [Domain] | Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	0	75
Repurchase Agreements
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	2,700	2,310
Repurchase Agreements | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	2,700	2,310
Repurchase Agreements | Open (1) | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Repurchase Agreements | 1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	2,700	2,310
Repurchase Agreements | Over 1 to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Repurchase Agreements | Maturity 180 to 360 Days [Member] [Domain] | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	$ 0	$ 0