Quarterly report pursuant to Section 13 or 15(d)

4. Derivatives - Schedule of Derivatives Not Designated as Hedging Instruments (Details)

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4. Derivatives - Schedule of Derivatives Not Designated as Hedging Instruments (Details) - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Commodity contracts - Derivative assets $ 3,242 $ 2,081
Commodity contracts - Derivative liabilities 3,294 1,848
Commodity Contracts [Member] | Non Designated Derivative Instruments [Member]    
Commodity contracts - Derivative assets 3,242 2,081
Commodity contracts - Derivative liabilities $ 3,492 $ 1,848