Reinsurance (Reinsurance Recoverable by Category and Listing of Largest Reinsurers) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 17,695
Valuation allowance for Uncollectible Reinsurance	$ 329	$ 314	$ 316
% of Gross Reinsurance Recoverable	1.90%
Largest reinsurers
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 8,441
Valuation allowance for Uncollectible Reinsurance	$ 102
% of Gross Reinsurance Recoverable	1.20%
Other reinsurers rated A- or better
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 5,435
Valuation allowance for Uncollectible Reinsurance	$ 63
% of Gross Reinsurance Recoverable	1.20%
Other reinsurers rated lower than A- or not rated
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 315
Valuation allowance for Uncollectible Reinsurance	$ 48
% of Gross Reinsurance Recoverable	15.30%
Pools
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 415
Valuation allowance for Uncollectible Reinsurance	$ 10
% of Gross Reinsurance Recoverable	2.40%
Structured settlements
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 516
Valuation allowance for Uncollectible Reinsurance	$ 13
% of Gross Reinsurance Recoverable	2.50%
Captives
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 2,325
Valuation allowance for Uncollectible Reinsurance	$ 28
% of Gross Reinsurance Recoverable	1.20%
Other
Ceded Credit Risk [Line Items]
Gross Reinsurance Recoverable on Loss and Loss Expenses	$ 248
Valuation allowance for Uncollectible Reinsurance	$ 65
% of Gross Reinsurance Recoverable	26.20%