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, 2021	Sep. 30, 2020
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 760,561	$ 772,887
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	105,707	95,343
Net balance, beginning of period	654,854	677,544
Add incurred related to:
Current accident year	149,947	166,551
Prior accident years	(48,346)	(45,585)
Total incurred	101,601	120,966
Less paid related to:
Current accident year	30,023	27,986
Prior accident years	112,535	105,832
Total paid	142,558	133,818
Net balance, end of period	613,897	664,692
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	106,892	95,220
Balance, end of period	$ 720,789	$ 759,912