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Asset Retirement Obligation (Tables)
 3 Months Ended
Mar. 31, 2015
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,
 
2015
 
2014
Asset retirement obligation, beginning of period
$
54,970

 
$
48,536

Liabilities incurred
380

 
—

Liabilities settled
(894
)
 
(718
)
Accretion expense
859

 
791

Revisions in estimated cash flows
43

 
561

Asset retirement obligation, end of period
55,358

 
49,170

Less: current portion of asset retirement obligation
(1,910
)
 
(3,390
)
Long-term asset retirement obligation
$
53,448

 
$
45,780