Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	$ 3,023	$ 3,646
U.S. government and agency
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	3,023	3,646
U.S. government and agency | Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	1,446	1,474
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	686	1,823
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	$ 891	$ 349