Derivative Financial Instruments (Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Commodity futures, asset derivatives	$ 7,151	$ 2,790
Commodity futures, liability derivatives	1,407	10,506
Futures | Commodity contract | Designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	4,918	1,211
Commodity futures, liability derivatives	555	5,364
Futures | Commodity contract | Not designated as hedging instrument | Other current assets
Commodity futures, asset derivatives	2,233	1,579
Commodity futures, liability derivatives	$ 852	$ 5,142