Derivative Instruments - Derivatives Not Designated as Hedging Instruments by Classification and Amount of Gains and Losses (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) recognized in the statements of operations for derivatives not designated as hedging instruments	$ (23,400)	$ 70,700	$ (23,353)	$ 70,711	$ (9,425)
Foreign currency forward exchange contracts | Other non-operating expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) recognized in the statements of operations for derivatives not designated as hedging instruments			(23,109)	70,200	(11,056)
Foreign currency forward exchange contracts | Cost of sales
Derivative Instruments, Gain (Loss) [Line Items]
Net gain (loss) recognized in the statements of operations for derivatives not designated as hedging instruments			$ (244)	$ 511	$ 1,631