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Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of activity in asset retirement obligation
The following table presents the activity in the Company’s asset retirement obligation for the years ended December 31, 2012 and 2011 (in thousands):
 
December 31,
 
2012
 
2011
Balance at beginning of period
$
15,541

 
$
12,471

Obligations settled
(2,954
)
 
(1,030
)
Accretion expense
1,299

 
955

Revisions in estimated cash flows
7,872

 
2,508

Loss on settlement
367

 
637

Balance at end of period
$
22,125

 
$
15,541