Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 20,319
Valuation allowance for Uncollectible Reinsurance	$ 367	$ 351	$ 329
% of Gross Reinsurance Recoverable	1.80%
Largest reinsurers
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 10,993
Valuation allowance for Uncollectible Reinsurance	$ 117
% of Gross Reinsurance Recoverable	1.10%
Other reinsurers rated A- or better
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 4,898
Valuation allowance for Uncollectible Reinsurance	$ 57
% of Gross Reinsurance Recoverable	1.20%
Other reinsurers rated lower than A- or not rated
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 455
Valuation allowance for Uncollectible Reinsurance	$ 56
% of Gross Reinsurance Recoverable	12.30%
Pools
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 441
Valuation allowance for Uncollectible Reinsurance	$ 14
% of Gross Reinsurance Recoverable	3.20%
Structured settlements
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 493
Valuation allowance for Uncollectible Reinsurance	$ 11
% of Gross Reinsurance Recoverable	2.20%
Captives
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 2,653
Valuation allowance for Uncollectible Reinsurance	$ 16
% of Gross Reinsurance Recoverable	0.60%
Other
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 386
Valuation allowance for Uncollectible Reinsurance	$ 96
% of Gross Reinsurance Recoverable	24.90%