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