Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	$ 3,330	$ 3,674
Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	907	937
Remaining Tenor of Securities Lending Agreements: 1 Month or Less
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	1,709	2,300
Remaining Tenor of Securities Lending Agreements: 1 to 6 Months
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	714	437
U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	3,329	3,674
U.S. government and agency | Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	906	937
U.S. government and agency | Remaining Tenor of Securities Lending Agreements: 1 Month or Less
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	1,709	2,300
U.S. government and agency | Remaining Tenor of Securities Lending Agreements: 1 to 6 Months
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	714	437
U.S. corporate
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	1	0
U.S. corporate | Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	1	0
U.S. corporate | Remaining Tenor of Securities Lending Agreements: 1 Month or Less
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	0	0
U.S. corporate | Remaining Tenor of Securities Lending Agreements: 1 to 6 Months
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	$ 0	$ 0