Impairments (Details) (USD $)	3 Months Ended				12 Months Ended						3 Months Ended	12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Appalachian	Dec. 31, 2011 Appalachian	Dec. 31, 2012 Marcellus Shale	Jun. 30, 2011 Arkoma Basin	Dec. 31, 2011 Arkoma Basin	Dec. 31, 2011 Mississippian Properties
Significant Acquisitions and Disposals [Line Items]
Oil and gas properties					$ 103,417,000	$ 104,688,000	$ 43,067,000	$ 28,400,000		$ 75,000,000
Other - tubular inventory and well materials					1,067,000	0	2,892,000
Total impairments					104,484,000	104,688,000	45,959,000
Impairement of oil and gas properties	$ 75,200,000	$ 700,000	$ 28,600,000	$ 33,600,000					$ 26,600,000		$ 71,000,000	$ 71,100,000	$ 6,800,000