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Asset Retirement Obligation (Tables)
12 Months Ended
Dec. 31, 2011
Asset Retirement Obligation  
Schedule of reconciliation of the changes in the asset retirement obligation
 
  December 31, 2011   December 31, 2010  

Beginning asset retirement obligation

  $ 4,029   $ 2,877  

Liabilities incurred

    1,599     915  

Accretion expense

    1,190     237  
           

Ending asset retirement obligation

  $ 6,818   $ 4,029