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Derivative Instruments and Hedging Activities - Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) - Designated as Hedging Instrument [Member] - Commodity Contracts [Member] - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Asset Derivatives    
Fair Value of Derivative Instruments, Assets $ 38,576 $ 141,366
Liability Derivatives    
Fair Value of Derivative Instruments, Liabilities 0 0
Current liabilities [Member]    
Liability Derivatives    
Fair Value of Derivative Instruments, Liabilities 0 0
Long-term liabilities [Member]    
Liability Derivatives    
Fair Value of Derivative Instruments, Liabilities 0 0
Current Asset [Member]    
Asset Derivatives    
Fair Value of Derivative Instruments, Assets 38,576 127,033
Long-term assets [Member]    
Asset Derivatives    
Fair Value of Derivative Instruments, Assets $ 0 $ 14,333