Asset Retirement Obligations (Asset Retirement Obligations) (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010
Asset Retirement Obligations
Beginning asset retirement obligations	$ 154,689,000	$ 142,270,000		$ 152,291,000		$ 166,434,000
Liabilities assumed in acquisitions		6,000		6,000		6,000
New wells placed on production	1,404,000	4,365,000		2,075,000		4,644,000
Changes in estimates	(179,000)			121,000
Disposition of wells	(367,000)	(3,291,000)		(448,000)		(29,540,000)
Liabilities settled	(6,235,000)	(8,801,000)		(7,469,000)		(9,955,000)
Accretion of discount from continuing operations	2,658,000	2,529,000		5,313,000		5,388,000
Accretion of discount from discontinued operations		103,000	[1]	81,000	[1]	204,000	[1]
Ending asset retirement obligations	151,970,000	137,181,000		151,970,000		137,181,000
Asset retirement obligations, current portion	$ 14,900,000			$ 14,900,000				$ 19,900,000

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