Reinsurance - Consolidated Property Casualty Insurance Assumed and Ceded Business (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Property Casualty Insurance Assumed And Ceded Business
Earned premiums	$ 2,033	$ 1,884	$ 5,894	$ 5,350
Incurred loss and loss expenses	1,332	1,418	4,070	3,772
Consolidated Property And Casualty Insurance
Property Casualty Insurance Assumed And Ceded Business
Direct written premiums	1,939	1,730	5,869	5,304
Assumed written premiums	89	92	527	548
Ceded written premiums	(71)	(72)	(270)	(239)
Net written premiums	1,957	1,750	6,126	5,613
Direct earned premiums	1,912	1,743	5,478	4,935
Assumed earned premiums	142	161	435	416
Ceded earned premiums	(97)	(95)	(252)	(227)
Earned premiums	1,957	1,809	5,661	5,124
Direct incurred loss and loss expenses	1,220	1,247	3,784	3,316
Assumed incurred loss and loss expenses	82	164	216	304
Ceded incurred loss and loss expenses	(41)	(63)	(160)	(76)
Incurred loss and loss expenses	$ 1,261	$ 1,348	$ 3,840	$ 3,544