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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation [Abstract]  
Changes to Asset Retirement Obligation

The following table describes changes to the asset retirement obligation liability (dollars in millions):

 

 

 

Year Ended December 31

 

 

 

2017

 

 

2016

 

Asset retirement obligation at beginning of period

 

$

28.0

 

 

$

26.2

 

Accretion expense

 

 

1.3

 

 

 

2.3

 

Payments

 

 

(2.3

)

 

 

(2.2

)

Revisions in estimated cash flows

 

 

7.9

 

 

 

1.2

 

Liabilities incurred

 

 

0.2

 

 

 

0.5

 

Asset retirement obligation at end of period

 

$

35.1

 

 

$

28.0