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Asset Retirement Obligation (Tables)
 3 Months Ended
Mar. 31, 2016
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,
 
2016
 
2015
Asset retirement obligation, beginning of period
$
42,556

 
$
54,970

Liabilities incurred
—

 
380

Liabilities settled
(464
)
 
(894
)
Accretion expense
608

 
859

Revisions in estimates
—

 
43

Asset retirement obligation, end of period
42,700

 
55,358

Less: current portion of asset retirement obligation
(5,545
)
 
(1,910
)
Long-term asset retirement obligation
$
37,155

 
$
53,448