Reinsurance Reinsurance (Written Consolidated Property Casualty Insurance Premiums On Assumed and Ceded Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-17, Insurance Companies, Reinsurance
Direct premiums earned	$ 7,801	$ 7,056	$ 6,366
Assumed premiums earned	569	561	443
Ceded premiums earned	(412)	(392)	(331)
Incurred loss and loss expenses	5,274	5,019	3,909
Consolidated Property And Casualty Insurance Entity
SEC Schedule, 12-17, Insurance Companies, Reinsurance
Direct premiums written	7,784	7,002	6,229
Assumed premiums written	597	619	515
Ceded premiums written	(335)	(314)	(265)
Net written premiums	8,046	7,307	6,479
Direct premiums earned	7,407	6,677	5,996
Assumed premiums earned	569	561	443
Ceded premiums earned	(331)	(314)	(255)
Earned premiums	7,645	6,924	6,184
Direct incurred loss and loss expenses	4,843	4,495	3,352
Assumed incurred loss and loss expenses	280	396	366
Ceded incurred loss and loss expenses	(165)	(175)	(122)
Incurred loss and loss expenses	$ 4,958	$ 4,716	$ 3,596