Derivative Financial Instruments (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Inventories
Fair values of derivative financial instruments
Asset Derivatives	$ 0		$ 0
Liability Derivatives	0		0
Amount of Gain (Loss) Recognized in Income on Hedged Items	10,396,000	89,957,000
Derivatives in Fair Value Hedging Relationships | Futures contracts
Fair values of derivative financial instruments
Asset Derivatives	0		0
Liability Derivatives	0		0
Amount of Gain (Loss) Recognized in Income on Derivatives	$ (10,384,000)	$ (89,878,000)