Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	$ 3,674	$ 3,074
Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	1,289	1,279
Remaining Tenor of Securities Lending Agreements: 1 Month or Less
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	2,012	1,094
Remaining Tenor of Securities Lending Agreements: 1 to 6 Months
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	373	701
U.S. Government and Agency
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	3,669	3,074
U.S. Government and Agency | Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	1,284	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	2,012	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	373	701
U.S. Corporate
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	2	0
U.S. Corporate | Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	2	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
Foreign Corporate
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	3	0
Foreign Corporate | Remaining Tenor of Securities Lending Agreements: Open
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	3	0
Foreign Corporate | Remaining Tenor of Securities Lending Agreements: 1 Month or Less
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	0	0
Foreign Corporate | Remaining Tenor of Securities Lending Agreements: 1 to 6 Months
Securities Financing Transaction [Line Items]
Cash collateral received from counterparties	$ 0	$ 0