Derivative Financial Instruments (Schedule of Derivatives Instruments Statements of Financial Position) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Designated as hedging instrument
Amount of gain (loss) recognized in income on related hedged items	$ 4,822	$ (4,439)	$ 3,353
Designated as hedging instrument | Firm commitments | Cost of goods sold
Amount of gain (loss) recognized in income on related hedged items	1,814	(3,901)	2,084
Designated as hedging instrument | Inventory | Cost of goods sold
Amount of gain (loss) recognized in income on related hedged items	3,008	(538)	1,269
Futures | Designated as hedging instrument | Commodity contract | Cost of goods sold
Amount of gain (loss) recognized in income on derivatives	5,763	3,596	(4,254)
Futures | Not designated as hedging instrument | Commodity contract | Cost of goods sold
Amount of gain (loss) recognized in income on derivatives	$ (18,784)	$ (8,450)	$ 16,261