Loss Reserves - Liability for Unpaid Loss and Loss Adjustment Expenses, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	62,426	62,729
Prior accident years	(9,264)	(6,513)
Total incurred	53,162	56,216
Less paid related to:
Current accident year	4,686	3,206
Prior accident years	47,354	45,498
Total paid	52,040	48,704
Net balance, end of period	688,078	672,032
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	89,220	77,464
Balance, end of period	$ 777,298	$ 749,496