Transactions with Related Parties (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) director	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2024	Jul. 01, 2021	Jan. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)
Related Party Transaction [Line Items]
Purchases of other investments	$ 2,171,000,000	$ 1,720,000,000	$ 1,859,000,000
Net reserves	16,062,000,000	13,752,000,000	12,158,000,000					$ 12,199,000,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments	7,064,000,000	6,564,000,000	5,881,000,000
Ceded premiums written	4,935,000,000	4,249,000,000	3,735,000,000
Reinsurance balances payable	2,000,000,000	1,530,000,000
Net premiums written	13,468,000,000	11,078,000,000	9,017,000,000
Net premiums earned	$ 12,440,000,000	9,679,000,000	$ 8,082,000,000
Forecast
Related Party Transaction [Line Items]
Put Call Right To Sell Purchase Shares					33.30%
Premia Holdings Ltd
Related Party Transaction [Line Items]
Number of directors | director	7
Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership						40.00%
Kelso And Company | Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership						30.00%
Warburg Pincus LLC | Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership						30.00%
Equity Method Investee | Premia Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership				25.00%
Purchases of other investments				$ 100,000,000
Number of directors | director	2
Net reserves							$ 380,000,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments	$ 0	0
Funds held liability	0	0
Net premiums written	80,000,000	121,000,000
Net premiums earned	81,000,000	120,000,000
Funds Held under Reinsurance Agreements, Asset	158,000,000	119,000,000
Equity Method Investee | Somers Holdings Ltd.
Related Party Transaction [Line Items]
Reinsurance recoverable for paid and unpaid claims and claims adjustments	1,300,000,000	1,200,000,000
Reinsurance balances payable	475,000,000	414,000,000
Senior notes	$ 35,000,000
Preferred shares, dividend rate (as a percent)	6.50%
Net premiums written	$ 574,000,000	$ 519,000,000