Reinsurance (Reinsurance Recoverable on Ceded Reinsurance) (Details) - USD ($) $ in Millions		Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Reinsurance Disclosures [Abstract]
Reinsurance recoverable on unpaid losses and loss expenses	[1]	$ 17,163	$ 17,884	$ 16,520	$ 17,086
Reinsurance recoverable on paid losses and loss expenses	[1]	1,946	2,068
Reinsurance recoverable on losses and loss expenses	[1]	19,109	19,952
Reinsurance recoverable on policy benefits	[1]	272	280
Reinsurance Recoverables on Unpaid Losses, Allowance		290	285
Reinsurance Recoverables on Paid Losses, Allowance		85	82
Reinsurance Recoverable, Allowance for Credit Loss, Total		375	367	$ 351	$ 351
Reinsurance Recoverables on Future Policy Benefits, Allowance		$ 0	$ 0

[1] Net of valuation allowance for uncollectible reinsurance.