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, 2020	Sep. 30, 2019
Roll Forward In Liability For Unpaid Claims And Claims Adjustment Expense [Abstract]
Balance, beginning of period	$ 772,887	$ 798,409
Less amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	95,343	107,216
Net balance, beginning of period	677,544	691,193
Add incurred related to:
Current accident year	166,551	181,693
Prior accident years	(45,585)	(38,886)
Total incurred	120,966	142,807
Less paid related to:
Current accident year	27,986	35,512
Prior accident years	105,832	105,590
Total paid	133,818	141,102
Net balance, end of period	664,692	692,898
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	95,220	97,914
Balance, end of period	$ 759,912	$ 790,812