Derivative Financial Instruments - Fair Values of Outstanding Zero Cost Collar and Forward Contracts Recorded as Assets and Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Zero Cost Collars [Member]
Asset Derivatives:
Derivatives designated as hedging instruments, Asset		$ 2,827
Liability Derivatives:
Other current liabilities	$ (684)
Forward [Member]
Asset Derivatives:
Derivatives designated as hedging instruments, Asset		2,352
Liability Derivatives:
Other current liabilities	(1,911)
Other Current Assets [Member] | Zero Cost Collars [Member]
Asset Derivatives:
Derivatives designated as hedging instruments, Asset		2,827
Other Current Assets [Member] | Forward [Member]
Asset Derivatives:
Derivatives designated as hedging instruments, Asset		$ 2,352
Other Current Liabilities [Member] | Zero Cost Collars [Member]
Liability Derivatives:
Other current liabilities	684
Other Current Liabilities [Member] | Forward [Member]
Liability Derivatives:
Other current liabilities	$ 1,911