Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 19,252
Valuation allowance for Uncollectible Reinsurance	$ 351	$ 329	$ 314
% of Gross Reinsurance Recoverable	1.80%
Largest reinsurers
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 10,826
Valuation allowance for Uncollectible Reinsurance	$ 114
% 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	$ 4,161
Valuation allowance for Uncollectible Reinsurance	$ 47
% of Gross Reinsurance Recoverable	1.10%
Other reinsurers rated lower than A- or not rated
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 443
Valuation allowance for Uncollectible Reinsurance	$ 70
% of Gross Reinsurance Recoverable	15.80%
Pools
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 425
Valuation allowance for Uncollectible Reinsurance	$ 14
% of Gross Reinsurance Recoverable	3.30%
Structured settlements
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Losses and Loss Expenses	$ 507
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	$ 2,455
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	$ 435
Valuation allowance for Uncollectible Reinsurance	$ 82
% of Gross Reinsurance Recoverable	18.80%