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Asset Retirement Obligation (Tables)
 9 Months Ended
Sep. 30, 2014
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):
 
Nine Months Ended September 30,
 
2014
 
2013
Asset retirement obligation, beginning of period
$
48,536

 
$
27,260

Liabilities incurred
224

 
498

Liabilities assumed
—

 
15,319

Liabilities settled
(2,902
)
 
(2,415
)
Accretion expense
2,223

 
1,203

Revisions in estimated cash flows
743

 
987

Asset retirement obligation, end of period
48,824

 
42,852

Less: current portion of asset retirement obligation
(1,426
)
 
(1,502
)
Long-term asset retirement obligation
$
47,398

 
$
41,350