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) - Commodity Contracts [Member] - Non Designated Derivative Instruments [Member] - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]    
Realized Gains (Losses) $ (2,344) $ 908
Unrealized Gains (Losses) $ 2,560 $ (326)