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Asset Retirement Obligation
 3 Months Ended
Mar. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligation
Asset Retirement Obligation

The following table describes the changes to the Company’s asset retirement obligation liability (in thousands):
 
Three Months Ended March 31,
 
2018
 
2017
Asset retirement obligation, beginning of period
$
31,310

 
$
36,610

Liabilities incurred
7

 
13

Liabilities settled
(57
)
 
(540
)
Accretion expense
198

 
547

Revisions in estimates
(2
)
 
(381
)
Divestiture of oil and gas properties
(28,214
)
 
—

Asset retirement obligation, end of period
3,242

 
36,249

Less: current portion of asset retirement obligation
(938
)
 
(3,291
)
Long-term asset retirement obligation
$
2,304

 
$
32,958



The divestiture of oil and gas properties during 2018 totaling $28.2 million relates to the sale of the Company's Gulf of Mexico assets. The liabilities incurred, revisions in estimated cash flows and divestitures represent non-cash investing activities for purposes of the statement of cash flows.