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Summary of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2023
Accounting Policies [Abstract]  
Summary of Asset Retirement Obligation Liabilities

The following table describes changes to our asset retirement obligation liabilities:

 

December 31,

 

 

 

2023

 

 

2022

 

(Dollars in millions)

 

 

 

 

 

 

Asset retirement obligation at beginning of year

 

$

15.5

 

 

$

13.2

 

Accretion expense

 

 

1.1

 

 

 

1.1

 

Revision in estimated cash flows

 

 

(0.8

)

 

 

2.2

 

Cash expenditures

 

 

(0.6

)

 

 

(1.0

)

Balance at end of year

 

$

15.2

 

 

$

15.5