Impairment (Details) (USD $)	12 Months Ended					3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2009	Dec. 31, 2011 Raton Basin [Member]	Dec. 31, 2011 Barnett Shale Fields [Member]	Dec. 31, 2011 South Texas Edwards And Austin Chalk [Member]	Mar. 31, 2009 Uinta/Piceance [Member]	Dec. 31, 2009 Uinta/Piceance [Member]
Impairment [Line Items]
Asset group carrying value			$ 2,300,000,000	$ 456,800,000
Impairment of oil and gas properties	$ 354,408,000	$ 21,091,000			$ 354,400,000	$ 21,100,000	$ 21,100,000