Investments (Securities Lending, Repurchase Agreements and FHLB Agreements Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	$ 18,628	$ 17,369
U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	17,508	16,267
Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	1,117	1,026
U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	3	0
RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	76
Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	0	800
Open (1)
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	2,949	2,928
Open (1) | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	2,946	2,928
Open (1) | Foreign government
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	3	0
Open (1) | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	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	10,844	7,011
1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	10,553	6,676
1 Month or Less | Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	291	259
1 Month or Less | U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
1 Month or Less | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	76
1 Month or Less | Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	0	250
Over 1 Month to 6 Months
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	4,835	7,430
Over 1 Month to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	4,009	6,663
Over 1 Month to 6 Months | Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	826	767
Over 1 Month to 6 Months | U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 1 Month to 6 Months | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 1 Month to 6 Months | Municipals
Securities Financing Transaction [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	0	475
Over 6 Months to 1 Year
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 6 Months to 1 Year | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 6 Months to 1 Year | Foreign government
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 6 Months to 1 Year | U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 6 Months to 1 Year | RMBS
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Over 6 Months to 1 Year | 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	3,210	2,310
Repurchase Agreements | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	3,210	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	3,210	2,310
Repurchase Agreements | Over 1 Month to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	0	0
Repurchase Agreements | Over 6 Months to 1 Year | U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	$ 0	$ 0