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
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
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	$ 1,131,700,000	$ 1,399,214,000	$ 1,534,779,000	$ 1,399,214,000	$ 1,386,061,000
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	122,945,000		113,450,000	251,749,000	216,816,000
Prior years	(1,576,000)		(3,450,000)	5,228,000	166,684,000
Total incurred losses and loss and adjustment expenses	121,369,000		110,000,000	256,977,000	383,500,000
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	6,832,000		7,263,000	9,606,000	10,456,000
Prior years	92,765,000		126,410,000	193,620,000	247,999,000
Total loss and loss adjustment expense payments	99,597,000		133,673,000	203,226,000	258,455,000
Deduct: Loss reserves ceded in Retrocession Agreement	1,570,885,000			1,570,885,000		$ 1,348,628,000
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	1,159,145,000	1,131,700,000	1,511,106,000	1,159,145,000	1,511,106,000
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	1,571,486,000		935,896,000	1,571,486,000	935,896,000
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	2,730,631,000		2,447,002,000	2,730,631,000	2,447,002,000
Casualty Reinsurance
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Prior years	0		5,000,000		7,500,000
Loss Portfolio Transfer | Casualty Reinsurance
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Deduct: Loss reserves ceded in Retrocession Agreement		$ 299,493,000
Add: Changes in reinsurance recoverable of Retrocession Agreement unrelated to net reserve activity	$ 5,673,000		$ 0	$ 5,673,000	$ 0