Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 20,087
Valuation allowance for Uncollectible Reinsurance	$ 310	$ 367	$ 351
% of Gross Reinsurance Recoverable	1.50%
Largest reinsurers
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 10,278
Valuation allowance for Uncollectible Reinsurance	$ 116
% 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	$ 5,557
Valuation allowance for Uncollectible Reinsurance	$ 67
% 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	$ 441
Valuation allowance for Uncollectible Reinsurance	$ 27
% of Gross Reinsurance Recoverable	6.10%
Pools
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 422
Valuation allowance for Uncollectible Reinsurance	$ 10
% of Gross Reinsurance Recoverable	2.40%
Structured settlements
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 489
Valuation allowance for Uncollectible Reinsurance	$ 8
% of Gross Reinsurance Recoverable	1.60%
Captives
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 2,704
Valuation allowance for Uncollectible Reinsurance	$ 13
% of Gross Reinsurance Recoverable	0.50%
Other
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 196
Valuation allowance for Uncollectible Reinsurance	$ 69
% of Gross Reinsurance Recoverable	35.20%