Loss and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Reserve Balances, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 718,033	$ 687,602	$ 614,557
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	64,858	59,334	48,699
Net balance, beginning of period	653,175	628,268	565,858
Add incurred related to:
Current accident year	250,337	262,387	268,633
Prior accident years	(51,306)	(47,814)	(23,717)
Total incurred	199,031	214,573	244,916
Less paid related to:
Current accident year	52,085	53,955	52,848
Prior accident years	135,601	135,711	129,658
Total paid	187,686	189,666	182,506
Net balance, end of period	664,520	653,175	628,268
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	78,256	64,858	59,334
Balance, end of period	$ 742,776	$ 718,033	$ 687,602