Net Reserves for Asbestos and Environmental Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Net reserve for A&E losses and loss adjustment expenses - beginning of period	$ 29,033	$ 29,524	$ 30,124
Plus: Change in incurred losses and loss adjustment expenses	1	(1)	0
Less: Payments	355	490	600
Net reserves for A&E losses and loss adjustment expenses - end of period	$ 28,679	$ 29,033	$ 29,524