Reserve for Losses and Loss Adjustment Expenses - Reserve for Losses and Loss Adjustment Expenses, Net of Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 1,194,088	$ 989,825	$ 761,128
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	603,094	582,604	533,905
Prior years	69,008	17,672	21,472
Total incurred losses and loss and adjustment expenses	672,102	600,276	555,377
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	75,249	86,355	103,205
Prior years	413,480	309,658	223,475
Total loss and loss adjustment expense payments	488,729	396,013	326,680
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	1,377,461	1,194,088	989,825
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	668,045	467,371	302,524
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	$ 2,045,506	$ 1,661,459	$ 1,292,349