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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Asset Retirement Obligations

The following table presents the changes in the net asset retirement obligations for the years ended December 31, 2016 and 2015 (in millions):

 

     December 31,  
     2016      2015  

Net asset retirement obligation at January 1

    $ 26.4      $ 23.8  

Liabilities incurred

      1.0        1.1  

Liabilities settled

      (1.2 )       —    

Accretion expense

      1.6        1.5  
  

 

 

     

 

 

  

Net asset retirement obligation at December 31

    $ 27.8      $ 26.4