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Derivative Financial Instruments (Narrative) (Details) - Futures - Commodity contract - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Other current assets
Commodity contract commitments
Fair value of derivatives including required margin deposits	$ 2,400		$ 2,400		$ 3,200
Designated as hedging instrument
Gains or losses on derivative instruments, statement of income
Ineffectiveness - amount of gain (loss) recognized in income on related hedged items	49	$ (47)	97	$ (91)
Gain (loss) excluded from hedge effectiveness testing	$ (266)	$ (97)	$ 567	$ (65)