Income Taxes - Components of Comprehensive Provision for Income Taxes Allocation (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive provision for income taxes (benefit) allocable to:
Net income (loss)	$ (12,588)	$ 20,278	$ (9,258)	$ 22,457
Other comprehensive income (loss):
Marketable securities		(1,600)		(2,500)
Currency translation	(1,600)	1,600	(900)	2,500
Interest rate swap		(100)
Total	(14,000)	20,500	(9,700)	23,200
Defined Benefit Pension Plans
Other comprehensive income (loss):
Defined benefit plans	$ 200	200	$ 500	$ 700
OPEB
Other comprehensive income (loss):
Defined benefit plans		$ 100