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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Summary of Asset Retirement Obligations
The following table provides a summary of the changes in the Company’s asset retirement obligations:
(in thousands)
Amount
Balance, December 31, 2016
$
9,370

Accretion expense
1,042

Balance, December 31, 2017
$
10,412