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Asset Retirement Obligation
 6 Months Ended
Jun. 30, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligation
Asset Retirement Obligation
The following table describes the changes to the Company’s asset retirement obligation liability (in thousands):
 
Six Months Ended June 30,
 
2014
 
2013
Asset retirement obligation, beginning of period
$
48,536

 
$
27,260

Liabilities incurred
224

 
498

Liabilities settled
(1,149
)
 
(94
)
Accretion expense
1,499

 
660

Revisions in estimated cash flows
742

 
986

Asset retirement obligation, end of period
49,852

 
29,310

Less: current portion of asset retirement obligation
(2,959
)
 
(3,823
)
Long-term asset retirement obligation
$
46,893

 
$
25,487