Transactions With Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Net premiums written	$ 4,047	$ 3,355	$ 11,913	$ 10,207
Net premiums earned	3,970	3,248	10,957	9,096
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	7,948	$ 6,821	7,948	6,821	$ 7,473	$ 7,064	$ 6,717	$ 6,564
Reinsurance balances payable	2,586		2,586			2,000
Other assets	5,855		5,855			4,761
Other liabilities	2,835		2,835			1,942
Watford Insurance Company
Related Party Transaction [Line Items]
Total consideration paid	$ 35
Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Put/call option, proportion of shares	33.30%
Other assets	$ 261		261
Other liabilities	261		261
Operating affiliates | Premia
Related Party Transaction [Line Items]
Net premiums written				80
Net premiums earned				80
Funds held asset	137		137			158
Operating affiliates | Somers
Related Party Transaction [Line Items]
Net premiums written			581	$ 457
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,600		1,600			1,300
Reinsurance balances payable	$ 546		$ 546			$ 475