Derivative Financial Instruments (Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Commodity futures, asset derivatives	$ 4,149	$ 4,060
Commodity futures, liability derivatives	893	2,083
Futures | Commodity contract | Designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	2,576	2,910
Commodity futures, liability derivatives	396	1,300
Futures | Commodity contract | Not designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	1,573	1,150
Commodity futures, liability derivatives	$ 497	$ 783