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, 2020	Mar. 31, 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	57,268	61,587
Prior accident years	(13,621)	(11,974)
Total incurred	43,647	49,613
Less paid related to:
Current accident year	2,984	3,381
Prior accident years	43,890	43,180
Total paid	46,874	46,561
Net balance, end of period	674,317	694,245
Add amounts recoverable from reinsurers on unpaid loss and loss adjustment expenses	104,966	106,335
Balance, end of period	$ 779,283	$ 800,580