Loss Reserves - Liability for Unpaid Claims and Claim Adjustment Expenses, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 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	178,562	187,819
Prior accident years	(27,524)	(41,406)
Total incurred	151,038	146,413
Less paid related to:
Current accident year	31,749	31,209
Prior accident years	108,284	108,485
Total paid	140,033	139,694
Net balance, end of period	675,525	659,894
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	79,501	76,382
Balance, end of period	$ 755,026	$ 736,276