Divestitures and Assets Held for Sale (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
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	$ 19,503		$ 19,503		$ 246
Asset retirement obligation associated with oil and gas properties held for sale (note 3)	749		749		1,277
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	192,432	123,067	523,610	343,805
Gain on divestiture activity related to assets held for use	(8,532)	190,728	(31,246)	245,662
Oil and Gas Properties [Member]
Assets Held for Sale
Depletion, depreciation, amortization, and asset retirement obligation liability accretion			1,700
Oil and Gas Properties [Member]
Assets Held for Sale
Gain on divestiture activity related to assets held for use			$ 33,200