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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Asset Retirement Obligations    
Beginning of period $ 84,810  
Liabilities incurred 4,009  
Liabilities settled (2,142)  
Disposition of wells (151)  
Accretion expense 3,668  
Change in estimate (5)  
End of period 90,189  
Less current portion (5,005) (5,005)
Long-term asset retirement obligations $ 85,184