Divestitures and Assets Held for Sale (Details) (USD $)	6 Months Ended		1 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 Rockies Mid-Continent Divestiture	Jan. 31, 2011 Rocky Mountain	Jul. 31, 2011 Marcellus shale	Aug. 31, 2011 LaSalle County
Divestitures and Assets Held for Sale
Cash received on divestiture of non-strategic assets	$ 97,952,000	$ 247,998,000	$ 35,700,000	$ 45,500,000	$ 80,000,000	$ 227,400,000
Estimated gain on divestiture of assets			28,500,000	27,200,000		196,100,000
Assets Held for Sale
Reasonably certain period within which sale will take place (in years)	1
Book value of assets held for sale, net of accumulated depletion, depreciation and amortization	$ 130,100,000