Derivative Financial Instruments - Fair Values of Outstanding Zero Cost Collar and Forward Contracts Recorded as Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Zero Cost Collars [Member]
Asset Derivatives:
Derivatives designated as hedging instruments, Asset	$ 2,827
Liability Derivatives:
Derivatives designated as hedging instruments, Liability		$ 453
Forward [Member]
Asset Derivatives:
Derivatives designated as hedging instruments, Asset	2,352
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:
Derivatives designated as hedging instruments, Liability		$ 453