Divestitures and Assets Held for Sale (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 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	$ 60,700,000		$ 60,700,000
Asset retirement obligation associated with oil and gas properties held for sale (note 3)	1,732,000		1,732,000		1,277,000
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	161,608,000	115,382,000	331,178,000	220,738,000
Impairment of proved properties	38,523,000	0	38,523,000	0
Oil and Gas Properties [Member]
Assets Held for Sale
Depletion, depreciation, amortization, and asset retirement obligation liability accretion			1,700,000
Impairment of proved properties			$ 28,300,000