Gross Reserves for Asbestos and Environmental Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Gross reserve for A&E losses and loss adjustment expenses – beginning of period	$ 48,825	$ 50,445	$ 51,873
Plus: Change in incurred losses and loss adjustment expenses	(259)	(2)	(1)
Less: Payments	973	1,618	1,427
Gross reserves for A&E losses and loss adjustment expenses – end of period	$ 47,593	$ 48,825	$ 50,445