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

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Transactions with Related Parties (Details)	12 Months Ended
Transactions with Related Parties (Details)	Dec. 31, 2021 USD ($) director	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 01, 2021	Dec. 31, 2018 USD ($)
Related Party Transaction [Line Items]
Purchases of other investments	$ 1,859,096,000	$ 1,808,727,000	$ 1,470,545,000
Net reserves	12,157,925,000	12,199,074,000	9,809,192,000		$ 9,039,006,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments	5,880,735,000	4,500,802,000	4,346,816,000
Ceded premiums written	3,734,150,000	2,650,352,000	$ 2,099,893,000
Reinsurance balances payable	$ 1,583,253,000	683,263,000
Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership				40.00%
Premia Holdings Ltd
Related Party Transaction [Line Items]
Number of directors \| director	7
Greysbridge Holdings Ltd \| Kelso And Company
Related Party Transaction [Line Items]
Percentage ownership				30.00%
Greysbridge Holdings Ltd \| Warburg Pincus
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%
Purchases of other investments			$ 100,000,000
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	199,800,000
Funds held liability	0	$ 149,600,000
Equity Method Investee \| Somers Holdings Ltd.
Related Party Transaction [Line Items]
Reinsurance recoverable for paid and unpaid claims and claims adjustments	902,800,000
Ceded premiums written	453,900,000
Reinsurance balances payable	258,400,000
Senior notes	$ 35,000,000
Preferred shares, dividend rate (as a percent)	6.50%
Percentage ownership of affiliate preferred shares	6.60%