Income Taxes - Components of Comprehensive Provision for Income Taxes Allocation (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Comprehensive provision for income taxes (benefit) allocable to:
Net income (loss)	$ (6,158)	$ 6,466	$ (15,416)	$ 28,923
Other comprehensive income (loss):
Marketable securities		(2,800)		(5,300)
Currency translation	300	3,100	(600)	5,700
Interest rate swap		200		200
Total	(5,600)	7,300	(15,300)	30,500
Defined Benefit Pension Plans
Other comprehensive income (loss):
Defined benefit plans	300	400	800	1,100
OPEB
Other comprehensive income (loss):
Defined benefit plans	$ (100)	$ (100)	$ (100)	$ (100)