Loss Reserves - Change in Allowance for Credit Losses on Amounts Recoverable from Reinsurers (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Insurance [Abstract]
Balance, beginning of period	$ 449	$ 388	$ 452	$ 444
Provision for credit loss benefit	(60)	(9)	(63)	(65)
Balance, end of period	$ 389	$ 379	$ 389	$ 379