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Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Changes to the Company's asset retirement obligation liability
The following table summarizes the changes to the Company’s asset retirement obligation (in thousands):
 
Year Ended December 31,
 
2017
 
2016
Asset retirement obligation, beginning of period
$
36,610

 
$
42,556

Liabilities incurred
574

 
—

Liabilities settled
(3,364
)
 
(3,296
)
Accretion expense
2,252

 
2,515

Revisions in estimated cash flows
(4,514
)
 
(1,746
)
Divestiture of oil and gas properties
(248
)
 
(3,419
)
Asset retirement obligation, end of period
31,310

 
36,610

Less: current portion of asset retirement obligation
(687
)
 
(4,160
)
Long-term asset retirement obligation
$
30,623

 
$
32,450