Derivative Financial Instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Derivative Financial Instruments
Fair value of oil, natural gas, NGL commodity derivative markets, net liability	$ 68,700,000		$ 68,700,000		$ 52,300,000
Unrealized losses included in accumulated other comprehensive loss ("AOCL")	(10,900,000)		(10,900,000)		(18,600,000)
Unrealized losses included in accumulated other comprehensive loss ("AOCL"), net of income tax	(6,900,000)		(6,900,000)		(11,800,000)
Derivative losses relating to de-designated commodity hedges reclassified from AOCL to earnings net of tax	3,951,000	1,163,000	4,878,000	(782,000)
Crude oil
Derivative Financial Instruments
Anticipated production (in MMBbls)	8		8
Natural gas
Derivative Financial Instruments
Anticipated production (in MMBbls)	45,000,000		45,000,000
Natural gas liquids ("NGLs")
Derivative Financial Instruments
Anticipated production (in MMBbls)	1		1
De-designated Hedges
Derivative Financial Instruments
Unrealized losses included in accumulated other comprehensive loss ("AOCL")	(10,900,000)		(10,900,000)
Unrealized losses included in accumulated other comprehensive loss ("AOCL"), net of income tax	(6,900,000)		(6,900,000)
Derivative losses relating to de-designated commodity hedges reclassified from AOCL to earnings before tax effect			7,700,000
Derivative losses relating to de-designated commodity hedges reclassified from AOCL to earnings net of tax			4,900,000
Expected reclassification into earnings from AOCL after-tax net loss related to de-designated commodity derivative contracts			$ 6,900,000