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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2011
Asset Retirement Obligation Disclosure [Abstract] 
Future inflation factor used in the calculation of the asset retirement obligations2.00%
Credit adjusted risk-free interest rate for the calculation of the asset retirement obligations7.50%
Asset retirement obligations rollforward 
Asset retirement obligations, beginning of the period$ 42,945
Liabilities added from acquisitions6,966
Liabilities added from drilling917
Current year accretion expense2,816
Settlements(1,214)
Revision of estimate787
Asset retirement obligations, end of the period$ 53,217