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Derivative Financial Instruments (Narrative) (Details) - Commodity contract - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other current assets \| Futures
Commodity contract commitments
Fair value of derivatives including required margin deposits	$ 5,600	$ 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		(191)
Gain (loss) excluded from hedge effectiveness testing		$ (652)
Designated as hedging instrument \| Futures
Gains or losses on derivative instruments, statement of income
Ineffectiveness - amount of gain (loss) recognized in income on related hedged items	(3)		$ 90
Gain (loss) excluded from hedge effectiveness testing	$ (938)		$ (991)