Reserve for Losses and Loss Adjustment Expenses - Reconciliation of Losses and Loss Adjustment Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 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	$ 1,377,461	$ 1,194,088
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	95,982	138,959
Prior years	874	968
Total incurred losses and loss and adjustment expenses	96,856	139,927
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	4,271	4,679
Prior years	118,357	107,684
Total loss and loss adjustment expense payments	122,628	112,363
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	$ 1,377,461	$ 1,194,088	$ 1,351,689	$ 1,377,461	$ 1,221,652
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period			691,669	668,045	508,655
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period			$ 2,043,358	$ 2,045,506	$ 1,730,307