Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	$ 3,674	$ 3,074
U.S. Government and Agency
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	3,674	3,074
U.S. Government and Agency | Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	937	1,279
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	2,300	1,094
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	$ 437	$ 701