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Asset Retirement Obligations (Tables)
6 Months Ended
Jun. 30, 2012
Asset Retirement Obligations [Abstract]  
Schedule of change in asset retirement obligation
                 
    Six Months Ended June 30,  
    2011     2012  

Asset retirement obligations at beginning of period

  $ 7,150     $ 11,733  

Liabilities incurred

    44       75  

Liabilities settled

    —         (98 ) 

Accretion

    322       420  

Divestitures

    —         —    
   

 

 

   

 

 

 

Asset retirement obligations at end of period

  $ 7,516     $ 12,130