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Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Derivative assets, Estimated Fair Value $ 123 $ 1,569
Derivative liabilities, Estimated Fair Value (38) (1,306)
Other Assets [Member]    
Derivatives, Fair Value [Line Items]    
Derivative assets, Estimated Fair Value 123 1,569
Other Liabilities [Member]    
Derivatives, Fair Value [Line Items]    
Derivative liabilities, Estimated Fair Value $ (38) (1,306)
Oil [Member]    
Derivatives, Fair Value [Line Items]    
Notional Units Barrels  
Derivative assets, Notional Amount $ 38 83
Derivative assets, Estimated Fair Value 56 576
Derivative liabilities, Notional Amount (38) (83)
Derivative liabilities, Estimated Fair Value $ (17) (496)
Natural Gas [Member]    
Derivatives, Fair Value [Line Items]    
Notional Units MMBTUs  
Derivative assets, Notional Amount $ 350 1,900
Derivative assets, Estimated Fair Value 67 993
Derivative liabilities, Notional Amount (350) (1,900)
Derivative liabilities, Estimated Fair Value $ (21) $ (810)