Quarterly report pursuant to Section 13 or 15(d)

4. Derivatives - Schedule of Recognized Gains (Losses) for Derivatives (Details)

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4. Derivatives - Schedule of Recognized Gains (Losses) for Derivatives (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gains (Losses) $ (574) $ (999)    
Unrealized Gains (Losses) $ 6 $ 1,227    
Commodity Contracts [Member] | Non Designated Derivative Instruments [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Realized Gains (Losses)     $ (2,918) $ (91)
Unrealized Gains (Losses)     $ 2,566 $ 900