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Asset Retirement Obligations
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
Asset Retirement Obligations

A reconciliation of the Company's asset retirement obligations for the year ended December 31, 2018, 2017 and 2016 is as follows:

 
Year Ended December 31,
 
2018
 
2017
 
2016
 
(in thousands)
Beginning of period
$
17,586

 
$
11,238

 
$
15,176

Liabilities incurred (1)(2)
10,649

 
10,683

 
83

Liabilities settled
(1,630
)
 
(1,063
)
 
(16
)
Disposition of properties
(351
)
 
(5,138
)
 
(4,840
)
Accretion expense
1,291

 
972

 
861

Revisions to estimate
2,110

 
894

 
(26
)
End of period
$
29,655

 
$
17,586

 
$
11,238

Less: Current asset retirement obligations
2,325

 
1,489

 
535

Long-term asset retirement obligations
$
27,330

 
$
16,097

 
$
10,703



(1)
The year ended December 31, 2018 includes $7.4 million associated with properties acquired in the Merger. See Note 4 for additional information regarding the Merger.
(2)
The year ended December 31, 2017 includes $8.7 million associated with properties acquired in the DJ Basin. See Note 4 for additional information regarding this acquisition.