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Asset Retirement Obligation (Tables)
 9 Months Ended
Sep. 30, 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):
 
Nine Months Ended September 30,
 
2017
 
2016
Asset retirement obligation, beginning of period
$
36,610

 
$
42,556

Liabilities incurred
574

 
—

Liabilities settled
(2,486
)
 
(2,943
)
Accretion expense
1,671

 
1,896

Revisions in estimates
(161
)
 
291

Divestiture of oil and gas properties
(248
)
 
(75
)
Asset retirement obligation, end of period
35,960

 
41,725

Less: current portion of asset retirement obligation
(1,630
)
 
(1,132
)
Long-term asset retirement obligation
$
34,330

 
$
40,593