Derivative Instruments and Hedging Activities (Details) (Commodity contracts [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 47,720	$ 12,955
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(11,937)	(35,750)
Current assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	25,177	12,955
Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(11,122)	(32,144)
Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	22,543	0
Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	$ (815)	$ (3,606)