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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation [Abstract]  
Asset Retirement Obligations
The following table describes the changes to the Company’s asset retirement obligations liability for the following periods:
 
Year Ended December 31,
 
2018
 
2017
 
2016
 
(in thousands)
Asset retirement obligations, beginning of period
$
21,285

 
$
17,422

 
$
12,711

Additional liabilities incurred
2,843

 
1,526

 
637

Liabilities acquired
111,197

 
2,432

 
3,696

Liabilities settled
(1,788
)
 
(1,555
)
 
(711
)
Accretion expense
2,132

 
1,391

 
1,064

Revisions in estimated liabilities
572

 
69

 
25

Asset retirement obligations, end of period
136,241

 
21,285

 
17,422

Less current portion
60

 
1,163

 
1,288

Asset retirement obligations - long-term
$
136,181

 
$
20,122

 
$
16,134