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, 2020	Dec. 31, 2019	Dec. 31, 2018
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 772,887	$ 798,409	$ 771,845
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	95,343	107,216	84,889
Net balance, beginning of period	677,544	691,193	686,956
Add incurred related to:
Current accident year	220,710	241,344	250,487
Prior accident years	(63,484)	(65,002)	(45,596)
Total incurred	157,226	176,342	204,891
Less paid related to:
Current accident year	50,113	58,883	62,061
Prior accident years	129,803	131,108	138,593
Total paid	179,916	189,991	200,654
Net balance, end of period	654,854	677,544	691,193
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	105,707	95,343	107,216
Balance, end of period	$ 760,561	$ 772,887	$ 798,409