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Asset Retirement Obligation (Tables)
 3 Months Ended
Mar. 31, 2017
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,
 
2017
 
2016
Asset retirement obligation, beginning of period
$
36,610

 
$
42,556

Liabilities incurred
13

 
—

Liabilities settled
(540
)
 
(464
)
Accretion expense
547

 
608

Revisions in estimates
(381
)
 
—

Asset retirement obligation, end of period
36,249

 
42,700

Less: current portion of asset retirement obligation
(3,291
)
 
(5,545
)
Long-term asset retirement obligation
$
32,958

 
$
37,155