Derivative Instruments and Hedging Activities (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Effect of derivative instruments on the Condensed Consolidated Balance Sheet
Fair Value Asset	$ 50,824,000	$ 198,638,000
Fair Value Liability	(192,000)	(3,511,000)
Unrealized gains included in Accumulated Other Comprehensive Income, pretax	50,600,000	198,300,000
Unrealized gains included in Accumulated Other Comprehensive Income, net of tax	30,700,000	121,300,000
Expected reclassification of income from commodity hedges over the next 12 months	30,700,000
Derivatives Designated as Hedging Instruments
Effect of derivative instruments on the Condensed Consolidated Balance Sheet
Fair Value Asset	50,824,000	198,638,000
Fair Value Liability	(192,000)	(385,000)
Derivatives Designated as Hedging Instruments | Commodity contracts | Current Assets
Effect of derivative instruments on the Condensed Consolidated Balance Sheet
Fair Value Asset	50,824,000	177,389,000
Derivatives Designated as Hedging Instruments | Commodity contracts | Accrued Liabilities
Effect of derivative instruments on the Condensed Consolidated Balance Sheet
Fair Value Liability	(192,000)	(385,000)
Derivatives Designated as Hedging Instruments | Commodity contracts | non-current assets
Effect of derivative instruments on the Condensed Consolidated Balance Sheet
Fair Value Asset		21,249,000
Derivatives Not Designated as Hedging Instruments | Commodity contracts | Current Assets
Effect of derivative instruments on the Condensed Consolidated Balance Sheet
Fair Value Liability		$ (3,126,000)