Net 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]
Net reserve for A&E losses and loss adjustment expenses-beginning of period	$ 30,529	$ 31,185	$ 23,038
Less: Payments	1,145	657	2,848
Net reserves for A&E losses and loss adjustment expenses-end of period	29,890	30,529	31,185
Case Reserves
Statutory Reserve [Line Items]
Plus: Incurred losses and loss adjustment expenses	(125)	395	2,754
IBNR Reserves
Statutory Reserve [Line Items]
Plus: Incurred losses and loss adjustment expenses	$ 631	$ (394)	$ 8,241