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Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]  
Changes to the Company's asset retirement obligation liability
The following table describes all changes to the Company’s asset retirement obligation liability (in thousands):
 
Year Ended December 31,
 
2012
 
2011
Asset retirement obligation, beginning of period
$
30,427

 
$
24,592

Liabilities incurred
892

 
220

Liabilities settled
(2,627
)
 
(905
)
Accretion expense
2,078

 
2,049

Revisions in estimated cash flows
(3,510
)
 
4,471

Asset retirement obligation, end of period
27,260

 
30,427

Less: current portion of asset retirement obligation
(2,351
)
 
(3,110
)
Long-term asset retirement obligation
$
24,909

 
$
27,317