Transactions With Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Dec. 31, 2023
Related Party Transaction [Line Items]
Net premiums written	$ 3,964	$ 4,047	$ 12,827	$ 11,913
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	9,070	$ 7,948	9,070	7,948	$ 9,044	$ 8,260	$ 7,473	$ 7,064
Reinsurance balances payable	2,638		2,638			2,137
Other assets	6,440		6,440			6,231
Other liabilities	3,287		3,287			3,039
Operating affiliates | Premia
Related Party Transaction [Line Items]
Funds held asset	130		130			137
Operating affiliates | Somers
Related Party Transaction [Line Items]
Net premiums written			557	$ 581
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,900		1,900			1,600
Reinsurance balances payable	594		594			$ 489
Equity Method Investee | Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Other assets	242		242
Other liabilities	$ 242		$ 242