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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Changes to the Company's Before-tax Asset Retirement Obligation
The following table indicates the changes to the company’s before-tax asset retirement obligations in 2017, 2016 and 2015:
 
2017

 
 
2016

 
2015

Balance at January 1
$
14,243

 
 
$
15,642

 
$
15,053

Liabilities incurred
684

 
 
204

 
51

Liabilities settled
(1,721
)
 
 
(1,658
)
 
(981
)
Accretion expense
668

 
 
749

 
715

Revisions in estimated cash flows
340

 
 
(694
)
 
804

Balance at December 31
$
14,214

 
 
$
14,243

 
$
15,642