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, 2017	Dec. 31, 2016	Dec. 31, 2015
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 742,776	$ 718,033	$ 687,602
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	78,256	64,858	59,334
Net balance, beginning of period	664,520	653,175	628,268
Add incurred related to:
Current accident year	244,094	250,337	262,387
Prior accident years	(34,770)	(51,306)	(47,814)
Total incurred	209,324	199,031	214,573
Less paid related to:
Current accident year	56,951	52,085	53,955
Prior accident years	129,937	135,601	135,711
Total paid	186,888	187,686	189,666
Net balance, end of period	686,956	664,520	653,175
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	84,889	78,256	64,858
Balance, end of period	$ 771,845	$ 742,776	$ 718,033