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Asset Retirement Obligation (Tables)
 3 Months Ended
Mar. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Changes to the Company's asset retirement obligation liability
The following table describes the changes to the Company’s asset retirement obligation liability (in thousands):
 
Three Months Ended March 31,
 
2013
 
2012
Asset retirement obligation, beginning of period
$
27,260

 
$
30,427

Liabilities incurred
—

 
840

Liabilities settled
(72
)
 
(782
)
Accretion expense
332

 
500

Revisions in estimated cash flows
986

 
(42
)
Asset retirement obligation, end of period
28,506

 
30,943

Less: current portion of asset retirement obligation
(3,845
)
 
(2,302
)
Long-term asset retirement obligation
$
24,661

 
$
28,641