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, 2019	Jun. 30, 2018
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 798,409	$ 771,845
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	107,216	84,889
Net balance, beginning of period	691,193	686,956
Add incurred related to:
Current accident year	121,727	126,060
Prior accident years	(23,245)	(20,822)
Total incurred	98,482	105,238
Less paid related to:
Current accident year	15,926	18,386
Prior accident years	77,362	84,194
Total paid	93,288	102,580
Net balance, end of period	696,387	689,614
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	98,659	93,987
Balance, end of period	$ 795,046	$ 783,601