Loss and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Reserve Balances, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 798,409	$ 771,845	$ 742,776
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	107,216	84,889	78,256
Net balance, beginning of period	691,193	686,956	664,520
Add incurred related to:
Current accident year	241,344	250,487	244,094
Prior accident years	(65,002)	(45,596)	(34,770)
Total incurred	176,342	204,891	209,324
Less paid related to:
Current accident year	58,883	62,061	56,951
Prior accident years	131,108	138,593	129,937
Total paid	189,991	200,654	186,888
Net balance, end of period	677,544	691,193	686,956
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	95,343	107,216	84,889
Balance, end of period	$ 772,887	$ 798,409	$ 771,845