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Fair Value of Derivative Instruments on Condensed Consolidated Balance Sheets (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Derivative assets, current $ 6,140 $ 0
Derivative Asset, Noncurrent 2,520 0
Derivative assets 8,660 0
Commodity derivative liabilities – current 13,634 12,932
Derivative Liability, Noncurrent 4,923 14,437
Derivative Liability 18,557 27,369
Commodity contracts    
Derivatives, Fair Value [Line Items]    
Derivative Liability 8,700  
Commodity contracts | Derivative assets/liabilities - current    
Derivatives, Fair Value [Line Items]    
Derivative assets, current 6,140 0
Commodity derivative liabilities – current 13,634 12,932
Commodity contracts | Noncurrent Derivative Contract [Member]    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Noncurrent 2,520 0
Derivative Liability, Noncurrent $ 4,923 $ 14,437