Reserve for Losses and Loss Adjustment Expenses - Reconciliation of Losses and Loss Adjustment Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
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	$ 1,339,534	$ 1,237,930	$ 1,377,461	$ 1,194,088
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	101,921	157,113	295,523	440,810
Prior years	4,234	56,971	6,234	60,254
Total incurred losses and loss and adjustment expenses	106,155	214,084	301,757	501,064
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	6,950	21,021	13,558	46,275
Prior years	102,140	104,513	329,061	322,397
Total loss and loss adjustment expense payments	109,090	125,534	342,619	368,672
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	$ 1,339,534	$ 1,237,930	$ 1,377,461	$ 1,194,088	$ 1,336,599	$ 1,377,461	$ 1,326,480
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period					770,150		614,827
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period					$ 2,106,749	$ 2,045,506	$ 1,941,307