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, 2022	Mar. 31, 2021
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 745,278	$ 760,561
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	119,266	105,707
Net balance, beginning of period	626,012	654,854
Add incurred related to:
Current accident year	47,965	50,937
Prior accident years	(10,224)	(11,420)
Total incurred	37,741	39,517
Less paid related to:
Current accident year	2,850	2,361
Prior accident years	47,063	42,920
Total paid	49,913	45,281
Net balance, end of period	613,840	649,090
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	114,718	110,668
Balance, end of period	$ 728,558	$ 759,758