Derivative Financial Instruments (Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commodity futures, asset derivatives	$ 2,790	$ 4,060
Commodity futures, liability derivatives	10,506	2,083
Futures | Commodity contract | Designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	1,211	2,910
Commodity futures, liability derivatives	5,364	1,300
Futures | Commodity contract | Not designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	1,579	1,150
Commodity futures, liability derivatives	$ 5,142	$ 783