Loss Reserves - Liability for Unpaid Loss and Loss Adjustment Expenses, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 771,845	$ 742,776
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	84,889	78,256
Net balance, beginning of period	686,956	664,520
Add incurred related to:
Current accident year	126,060	119,828
Prior accident years	(20,822)	(17,184)
Total incurred	105,238	102,644
Less paid related to:
Current accident year	18,386	15,298
Prior accident years	84,194	80,571
Total paid	102,580	95,869
Net balance, end of period	689,614	671,295
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	93,987	78,464
Balance, end of period	$ 783,601	$ 749,759