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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Amount of funds held as security for payments to be received under reinsurance contracts.

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Amount of cash outflow to acquire investments classified as other.

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The percentage rate used to calculate dividend payments on preferred stock.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Transactions with Related Parties (Details)	12 Months Ended
Transactions with Related Parties (Details)	Dec. 31, 2022 USD ($) director	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2024	Jul. 01, 2021	Jan. 01, 2021 USD ($)	Dec. 31, 2019 USD ($)
Related Party Transaction [Line Items]
Purchases of other investments	$ 1,720,457,000	$ 1,859,096,000	$ 1,808,727,000
Net reserves	13,753,118,000	12,157,925,000	12,199,074,000					$ 9,809,192,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments	6,563,654,000	5,880,735,000	4,500,802,000
Ceded premiums written	4,249,258,000	3,734,150,000	2,650,352,000
Reinsurance balances payable	1,529,919,000	1,583,253,000
Net premiums written	11,077,189,000	9,018,337,000	7,437,716,000
Net premiums earned	$ 9,678,077,000	8,082,298,000	$ 6,991,935,000
Forecast
Related Party Transaction [Line Items]
Put Call Right To Sell Purchase Shares					33.30%
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%
Premia Holdings Ltd
Related Party Transaction [Line Items]
Number of directors \| director	7
Warburg Pincus LLC \| Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership						30.00%
Arch Re Bermuda And Affiliates \| Premia Holdings Ltd
Related Party Transaction [Line Items]
Number of directors \| director	2
Equity Method Investee \| Premia Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership				25.00%
Quota share agreement percentage	25.00%
Purchases of other investments				$ 100,000,000
Net premiums written	$ 121,100,000	40,400,000
Net premiums earned	119,900,000	40,400,000
Funds Held under Reinsurance Agreements, Asset	118,700,000	53,500,000
Equity Method Investee \| Barbican
Related Party Transaction [Line Items]
Net reserves							$ 380,000,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments	0	0
Funds held liability	0	0
Equity Method Investee \| Somers Holdings Ltd.
Related Party Transaction [Line Items]
Reinsurance recoverable for paid and unpaid claims and claims adjustments	1,200,000,000	902,800,000
Reinsurance balances payable	413,800,000	258,400,000
Senior notes	$ 35,000,000
Preferred shares, dividend rate (as a percent)	6.50%
Net premiums written	$ 519,400,000	$ 379,100,000