Income Taxes - Components of Comprehensive Provision for Income Taxes Allocation (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Comprehensive provision for income taxes (benefit) allocable to:
Net income	$ 3,330	$ 2,179
Other comprehensive income (loss):
Marketable securities		(900)
Currency translation	700	900
Interest rate swap		100
Total	4,300	2,700
Defined Benefit Pension Plans
Other comprehensive income (loss):
Defined benefit plans	$ 300	500
OPEB
Other comprehensive income (loss):
Defined benefit plans		$ (100)