Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2023 USD ($) lender	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($) lender	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Loss Contingencies [Line Items]
Current year		$ 779,650	$ 794,848	$ 813,515
(Favorable) Unfavorable development of reserves established in prior years, net		(40,215)	5,646	(36,753)
Prior years		(733,248)	$ (782,048)	(757,564)
Net favorable prior year reserve development		500
Net unfavorable development in workers' compensation business		300
Number of claims, net unfavorable prior year reserve development, from 1997 accident year | lender			1
Number of claims | lender	4
Net unfavorable prior year reserve development	$ 10,100
Lloyds Syndicates Operations
Loss Contingencies [Line Items]
Net unfavorable development medical professional liability business		6,800
MPL
Loss Contingencies [Line Items]
(Favorable) Unfavorable development of reserves established in prior years, net		(33,900)
Specialty P&C
Loss Contingencies [Line Items]
Current year		615,418	$ 624,481	648,531
(Favorable) Unfavorable development of reserves established in prior years, net		(36,932)	328	(22,486)
Prior year claims and claims adjustment expense, from covid 19				9,000
Specialty P&C | MTL
Loss Contingencies [Line Items]
Prior years		(4,500)
Portfolio Cell Reinsurance
Loss Contingencies [Line Items]
Net favorable prior year reserve development		2,800
Net favorable development in workers' compensation business		3,100
NORCAL Group
Loss Contingencies [Line Items]
Net reserves acquired from acquisitions					$ 1,100,000
Current year				4,900
(Favorable) Unfavorable development of reserves established in prior years, net		$ 5,300	$ 8,300	$ 10,800