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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2019
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,

 

 

 

2019

 

 

2018

 

Asset retirement obligation at beginning of period

 

$

30.0

 

 

$

35.1

 

Accretion expense

 

 

1.5

 

 

 

1.6

 

Payments

 

 

(3.5

)

 

 

(7.9

)

Revisions in estimated cash flows

 

 

(0.2

)

 

 

1.2

 

Asset retirement obligation at end of period

 

$

27.8

 

 

$

30.0