Derivative Instruments and Hedging Activities (Details 2) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	$ 47,402,000	$ 24,242,000
Derivative Assets, Noncurrent	29,000	12,919,000
Derivative Liabilities, Current	(26,523,000)	(78,184,000)
Derivative Liabilities, Noncurrent	(18,872,000)	(29,687,000)
Total Commodity Derivative Contracts	2,036,000	(70,710,000)
Crude Oil Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	23,452,000	3,050,000
Derivative Assets, Noncurrent	29,000	1,301,000
Derivative Liabilities, Current	(22,610,000)	(55,256,000)
Derivative Liabilities, Noncurrent	(18,702,000)	(25,906,000)
Natural Gas Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	23,950,000	21,192,000
Derivative Assets, Noncurrent	0	11,618,000
Derivative Liabilities, Current	0	0
Derivative Liabilities, Noncurrent	0	0
Deferred Premiums [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities, Current	(3,913,000)	(22,928,000)
Derivative Liabilities, Noncurrent	$ (170,000)	$ (3,781,000)