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, 2020	Jun. 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	112,342	121,727
Prior accident years	(31,165)	(23,245)
Total incurred	81,177	98,482
Less paid related to:
Current accident year	13,629	15,926
Prior accident years	78,303	77,362
Total paid	91,932	93,288
Net balance, end of period	666,789	696,387
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	101,978	98,659
Balance, end of period	$ 768,767	$ 795,046