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Financial Instruments - Fair value of financial assets and liabilties (Details) - Commodity Contract [Member] - Fair Value, Measurements, Recurring [Member] - Not Designated as Hedging Instrument [Member] - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Fair Value of Financial Assets and Liabilities    
Derivative Asset $ 0 $ 0
Derivative Liability 1,355 1,328
Fair Value, Inputs, Level 1 [Member]    
Fair Value of Financial Assets and Liabilities    
Derivative Asset, Fair Value, Gross Asset 63 68
Derivative Asset, Fair Value, Gross Liability (63) (68)
Derivative Liability, Fair Value, Gross Liability 1,418 1,396
Derivative Liability, Fair Value, Gross Asset $ (63) $ (68)