Derivative Financial Instruments (Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Commodity futures, asset derivatives	$ 4,060	$ 1,765
Commodity futures, liability derivatives	2,083	3,925
Futures | Commodity contract | Designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	2,910	857
Commodity futures, liability derivatives	1,300	2,860
Futures | Commodity contract | Not designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	1,150	908
Commodity futures, liability derivatives	$ 783	$ 1,065