Loss Reserves - Liability for Unpaid Loss and Loss Adjustment Expenses, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2022	Sep. 30, 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	145,993	149,947
Prior accident years	(30,201)	(48,346)
Total incurred	115,792	101,601
Less paid related to:
Current accident year	30,566	30,023
Prior accident years	111,968	112,535
Total paid	142,534	142,558
Net balance, end of period	599,270	613,897
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	116,926	106,892
Balance, end of period	$ 716,196	$ 720,789