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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2015
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 2015, 2014 and 2013:
 
2015

 
 
2014

 
2013

Balance at January 1
$
15,053

 
 
$
14,298

 
$
13,271

Liabilities incurred
51

 
 
133

 
59

Liabilities settled
(981
)
 
 
(1,291
)
 
(907
)
Accretion expense
715

 
 
882

 
627

Revisions in estimated cash flows
804

 
 
1,031

 
1,248

Balance at December 31
$
15,642

 
 
$
15,053

 
$
14,298