Reserve for Losses and Loss Adjustment Expenses - Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 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	$ 1,399,214	$ 1,386,061
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	128,804	103,366
Prior years	6,804	170,134
Total incurred losses and loss and adjustment expenses	135,608	273,500
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	2,774	3,194
Prior years	100,855	121,588
Total loss and loss adjustment expense payments	103,629	124,782
Deduct: Loss reserves ceded in Retrocession Agreement	1,617,884		$ 1,348,628
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	1,131,700	1,534,779
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	1,618,488	879,067
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	2,750,188	2,413,846
Casualty Reinsurance
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Prior years		2,500
Loss Portfolio Transfer | Casualty Reinsurance
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Deduct: Loss reserves ceded in Retrocession Agreement	$ 299,493	$ 0