Loss Reserves - Rollforward of Liability for Unpaid Claims and Claim Adjustment Expenses, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 742,776	$ 718,033
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	78,256	64,858
Net balance, beginning of period	664,520	653,175
Add incurred related to:
Current accident year	119,828	126,764
Prior accident years	(17,184)	(30,877)
Total incurred	102,644	95,887
Less paid related to:
Current accident year	15,298	12,479
Prior accident years	80,571	79,437
Total paid	95,869	91,916
Net balance, end of period	671,295	657,146
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	78,464	68,403
Balance, end of period	$ 749,759	$ 725,549