Ceiling Test (Details) (USD $)	3 Months Ended						12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Ceiling Test (Textual) [Abstract]
Increase (Decrease) in ceiling test write-downs	$ 28,100,000	$ 35,400,000	$ 53,500,000	$ 20,100,000	$ 13,000,000	$ 5,900,000	$ 137,100,000	$ 18,907,000	$ 0
Cash Flow Hedge, Decrease in Impairment of Gas and Oil Properties							$ (2,200,000)	$ (3,900,000)
Mcf of natural gas [Member]
Ceiling Test (Textual) [Abstract]
Average sales prices							2.21
Barrel of oil [Member]
Ceiling Test (Textual) [Abstract]
Average sales prices							102.81
Mcfe of Ngl [Member]
Ceiling Test (Textual) [Abstract]
Average sales prices							6.07