Reserve for Losses and Loss Adjustment Expenses - Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance (Details) - USD ($)	3 Months Ended			6 Months Ended		24 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) [Abstract]
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at beginning of period	$ 1,299,835,000	$ 1,131,700,000	$ 1,399,214,000	$ 1,248,270,000	$ 1,399,214,000
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	138,402,000	122,945,000		275,465,000	251,749,000
Prior years - excluding retroactive reinsurance	2,288,000	(1,576,000)		3,650,000	5,228,000
Total incurred losses and loss and adjustment expenses	141,308,000	121,369,000		296,596,000	256,977,000
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	5,859,000	6,832,000		8,711,000	9,606,000
Prior years	95,693,000	92,765,000		179,701,000	193,620,000
Total loss and loss adjustment expense payments	101,552,000	99,597,000		188,412,000	203,226,000
Deduct: Loss reserves ceded in Casualty Re LPT	1,545,736,000			1,545,736,000		$ 1,545,736,000	$ 1,520,113,000
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	1,338,973,000	1,159,145,000	$ 1,131,700,000	1,338,973,000	1,159,145,000	1,338,973,000
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	1,546,406,000	1,571,486,000		1,546,406,000	1,571,486,000	1,546,406,000
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	2,885,379,000	2,730,631,000		2,885,379,000	2,730,631,000	2,885,379,000
Excess and Surplus Lines
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Prior years - excluding retroactive reinsurance	118,000	(32,000)		(206,000)	(91,000)
Casualty Reinsurance
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Prior years - excluding retroactive reinsurance	3,000,000	0		4,900,000	6,800,000
Loss Portfolio Transfer | Excess And Surplus Lines and Casualty Reinsurance
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Prior years - excluding retroactive reinsurance	18,400,000			67,200,000
Loss Portfolio Transfer | Excess And Surplus Lines and Casualty Reinsurance | Commercial Auto Business
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Prior years - retroactive reinsurance	618,000	0		17,481,000	0
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Deduct: Change in deferred reinsurance gain - retroactive reinsurance	618,000	0		17,481,000	0
Loss Portfolio Transfer | Excess and Surplus Lines | Commercial Auto Business
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Prior years - excluding retroactive reinsurance	12,600,000			53,600,000		$ 100,300,000
Loss Portfolio Transfer | Casualty Reinsurance
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Deduct: Loss reserves ceded in Casualty Re LPT		299,493,000			299,493,000
Add: Changes in reinsurance recoverable of Casualty Re LPT unrelated to net reserve activity	$ 0	$ 5,673,000		$ 0	$ 5,673,000