Asset Retirement Obligations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation Roll Forward Analysis [Roll Forward]
Beginning asset retirement obligation	$ 85,744,000	$ 54,338,000
Liabilities incurred and assumed during period	12,477,000	4,291,000
Liabilities assumed in the Encore Merger	0	43,783,000
Revisions in estimated retirement obligations	12,217,000	5,505,000
Liabilities settled during period	(23,225,000)	(6,622,000)
Accretion expense	6,287,000	6,443,000
Sales of properties	(32,000)	(21,994,000)
Ending asset retirement obligation	93,468,000	85,744,000
Asset retirement obligations - current	(4,742,000)	(4,454,000)
Long-term asset retirement obligation	88,726,000	81,290,000
Asset Retirement Obligations (Textuals) [Abstract]
Balances in escrow accounts	$ 34,100,000	$ 33,100,000