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Accounting for Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2015
Accounting for Asset Retirement Obligations [Abstract]  
Reconciliation of asset retirement obligations
The following is a reconciliation of asset retirement obligations as of December 31, 2014 and 2013:

  
2015
  
2014
 
  
(millions of dollars)
 
Asset retirement liability, beginning of period
 
$
23.0
  
$
14.7
 
Acquisition
  
-
   
9.2
 
Accretion expense
  
2.8
   
1.7
 
Reversals
  
(1.0
)
  
(0.2
)
Payments
  
(1.9
)
  
(1.5
)
Foreign currency translation
  
(1.5
)
  
(0.9
)
Asset retirement liability,  end of period
 
$
21.4
  
$
23.0