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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
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	$ 839,508	$ 689,631	$ 761,128	$ 653,534
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	158,009	88,659	396,161	248,239
Prior years	(7,564)	(5,333)	(9,263)	(14,748)
Total incurred losses and loss and adjustment expenses	150,445	83,326	386,898	233,491
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	21,305	10,734	38,911	21,624
Prior years	65,349	43,847	205,816	147,025
Total loss and loss adjustment expense payments	86,654	54,581	244,727	168,649
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	903,299	718,376	903,299	718,376
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	283,949	156,286	283,949	156,286	$ 182,737
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	$ 1,187,248	$ 874,662	$ 1,187,248	$ 874,662	$ 943,865