Loss Reserves - Rollforward of Liability for Unpaid Claims and Claim Adjustment Expenses, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	62,729	65,154
Prior accident years	(6,513)	(18,438)
Total incurred	56,216	46,716
Less paid related to:
Current accident year	3,206	2,803
Prior accident years	45,498	41,766
Total paid	48,704	44,569
Net balance, end of period	672,032	655,322
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	77,464	66,856
Balance, end of period	$ 749,496	$ 722,178