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

 

     December 31,  
     2015      2014  

Net asset retirement obligation at January 1

    $ 23.8       $ 15.1   

Liabilities incurred

      1.1         4.6   

Acquisitions

      —           1.2   

Accretion expense

      1.5         1.1   

Changes in estimate

      —           1.8   
  

 

 

     

 

 

  

Net asset retirement obligation at December 31

    $ 26.4       $ 23.8