Loss Reserves - Liability for Unpaid Loss and Loss Adjustment Expenses, Net of Related Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 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	101,257	112,342
Prior accident years	(29,317)	(31,165)
Total incurred	71,940	81,177
Less paid related to:
Current accident year	12,786	13,629
Prior accident years	79,814	78,303
Total paid	92,600	91,932
Net balance, end of period	634,194	666,789
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	110,775	101,978
Balance, end of period	$ 744,969	$ 768,767