Investments (Securities Lending and Repurchase Agreements) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	$ 11,089	$ 10,788
Reinvestment portfolio - estimated fair value	10,861	10,553
Estimated fair value
Securities Financing Transaction [Line Items]
Securities loaned	10,851	10,510
Securities Sold under Agreements to Repurchase	3,032	3,029
Repurchase Agreements
Securities Financing Transaction [Line Items]
Cash collateral on deposit from counterparties	2,975	2,975
Reinvestment portfolio - estimated fair value	$ 2,912	$ 2,913