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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 791,781	$ 672,588	$ 761,128	$ 653,534
Add: Incurred losses and loss adjustment expenses net of reinsurance:
Current year	129,369	81,407	238,152	159,580
Prior years	1,715	(4,748)	(1,699)	(9,415)
Total incurred losses and loss and adjustment expenses	131,084	76,659	236,453	150,165
Deduct: Loss and loss adjustment expense payments net of reinsurance:
Current year	13,908	8,946	17,606	10,890
Prior years	69,449	50,670	140,467	103,178
Total loss and loss adjustment expense payments	83,357	59,616	158,073	114,068
Reserve for losses and loss adjustment expenses net of reinsurance recoverables at end of period	839,508	689,631	839,508	689,631
Add: Reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	221,553	153,706	221,553	153,706	$ 182,737
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	$ 1,061,061	$ 843,337	$ 1,061,061	$ 843,337	$ 943,865