Gross Reserves for Asbestos and Environmental Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statutory Reserve [Line Items]
Gross reserve for A&E losses and loss adjustment expenses-beginning of period	$ 53,824	$ 56,535	$ 50,155
Less: Payments	3,300	2,714	5,293
Gross reserves for A&E losses and loss adjustment expenses-end of period	51,919	53,824	56,535
Case Reserves
Statutory Reserve [Line Items]
Plus: Incurred losses and loss adjustment expenses	(669)	2,666	4,333
IBNR Reserves
Statutory Reserve [Line Items]
Plus: Incurred losses and loss adjustment expenses	$ 2,064	$ (2,663)	$ 7,340