Transactions With Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Net premiums written	$ 3,781	$ 3,428	$ 7,866	$ 6,852
Net premiums earned	3,565	2,965	6,987	5,848
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	7,473	6,717	7,473	6,717	$ 7,509	$ 7,064	$ 6,612	$ 6,564
Reinsurance balances payable	2,591		2,591			2,000
Operating affiliates | Premia
Related Party Transaction [Line Items]
Net premiums written		75
Net premiums earned		$ 75
Funds held asset	140		140			158
Operating affiliates | Somers
Related Party Transaction [Line Items]
Net premiums written			428	$ 314
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,400		1,400			1,300
Reinsurance balances payable	$ 542		$ 542			$ 475