Transactions With Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Related Party Transaction [Line Items]
Net premiums written	$ 3,355	$ 2,724	$ 10,207	$ 8,043
Net premiums earned	3,248	2,471	9,096	6,918
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	6,821	$ 6,356	6,821	6,356	$ 6,717	$ 6,564	$ 5,939	$ 5,881
Reinsurance balances payable	2,215		2,215			1,530
Operating affiliates | Somers
Related Party Transaction [Line Items]
Net premiums written			457	$ 552
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,300		1,300			1,200
Reinsurance balances payable	499		499			414
Equity Method Investee | Premia
Related Party Transaction [Line Items]
Net premiums written			80
Net premiums earned			80
Funds held asset	$ 176		$ 176			$ 119