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Asset Retirement Obligation (Tables)
 9 Months Ended
Sep. 30, 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):
 
Nine Months Ended September 30,
 
2015
 
2014
Asset retirement obligation, beginning of period
$
54,970

 
$
48,536

Liabilities incurred
392

 
224

Liabilities settled
(1,826
)
 
(2,902
)
Accretion expense
2,507

 
2,223

Revisions in estimates
(5,345
)
 
743

Divestiture of oil and gas properties
(2,103
)
 
—

Asset retirement obligation, end of period
48,595

 
48,824

Less: current portion of asset retirement obligation
(5,378
)
 
(1,426
)
Long-term asset retirement obligation
$
43,217

 
$
47,398