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Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Change in Asset Retirement Obligation
The table below summarizes the changes in our recorded asset retirement obligations (in thousands):
 
Year Ended December 31,
 
2018
 
2017
 
2016
Beginning balance
$
9,103

 
$
9,042

 
$
8,909

Obligations acquired
487

 
—

 
—

Accretion expense
395

 
369

 
362

Liabilities settled during period
—

 
(308
)
 
(229
)
Ending balance
$
9,985

 
$
9,103

 
$
9,042