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, 2021	Mar. 31, 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	50,937	57,268
Prior accident years	(11,420)	(13,621)
Total incurred	39,517	43,647
Less paid related to:
Current accident year	2,361	2,984
Prior accident years	42,920	43,890
Total paid	45,281	46,874
Net balance, end of period	649,090	674,317
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	110,668	104,966
Balance, end of period	$ 759,758	$ 779,283