Reserve for Losses and Loss Adjustment Expenses - Reconciliation of Losses and Loss Adjustment Balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
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	$ 1,221,652	$ 1,038,303	$ 1,194,088	$ 989,825
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	144,738	152,371	283,697	298,753
Prior years	2,315	2,224	3,283	(386)
Total incurred losses and loss and adjustment expenses	147,053	154,595	286,980	298,367
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	20,575	20,577	25,254	32,754
Prior years	110,200	79,503	217,884	162,620
Total loss and loss adjustment expense payments	130,775	100,080	243,138	195,374
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	1,237,930	1,092,818	1,237,930	1,092,818
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	545,404	375,535	545,404	375,535	$ 467,371
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	$ 1,783,334	$ 1,468,353	$ 1,783,334	$ 1,468,353	$ 1,661,459