Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Asset Retirement Obligations [Abstract]
Beginning asset retirement obligations	$ 152,291,000		$ 166,434,000		$ 172,433,000
Liabilities assumed in acquisitions	6,000		6,000
New wells placed on production	9,233,000		5,218,000		625,000
Changes in estimates	7,490,000	[1]	24,075,000	[1]	40,153,000	[1]
Liabilities reclassified to discontinued operations held for sale	(29,892,000)		(5,779,000)
Disposition of wells	(448,000)		(30,693,000)		(13,334,000)
Liabilities settled	(12,880,000)		(17,838,000)		(45,010,000)
Accretion of discount on continuing operations	8,256,000		7,945,000		8,050,000
Accretion of discount on discontinued operations	2,686,000	[2]	2,923,000	[2]	3,517,000	[2]
Ending asset retirement obligations	136,742,000		152,291,000		166,434,000
Asset retirement obligations, current portion	$ 14,200,000		$ 19,900,000

[1] The change in the 2011 and 2010 estimates are primarily due to increases in abandonment cost estimates based in part on recent actual costs incurred and a decline in credit-adjusted risk-free discount rates used to value increases in asset retirement obligations. These increases were partially offset by higher oil and NGL prices used to calculate proved reserves at December 31, 2011 and 2010, which had the effect of lengthening the economic life of certain wells and decreasing what would otherwise have been the present value of future retirement obligations. The increase in commodity prices was less substantial in 2011 as compared to 2010. The change in the 2009 estimate is primarily due to (i) lower gas prices used to calculate proved reserves at December 31, 2009, which had the effect of shortening the economic life of wells and increasing the present value of future retirement obligations primarily in the Raton, Hugoton and West Panhandle gas fields and (ii) a $19.9 million increase in East Cameron facilities reclamation and abandonment estimates.

[2] Represents the significant noncash components of discontinued operations.