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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2013
Asset Retirement Obligations  
Schedule of changes in the asset retirement obligations

 

 
   December 31,  
 
   2013   2012  

Balance at beginning of year

   $ 89,478   $ 74,963  

Accretion expense

     9,079     4,411  

Revisions in estimated cash flows

     26,453     14,353  

Obligations settled

     (8,617 )   (4,249 )
           
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Balance at end of year

   $ 116,393   $ 89,478  
           
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