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ASSET RETIREMENT OBLIGATION: (Tables)
9 Months Ended
Sep. 30, 2024
ASSET RETIREMENT OBLIGATION:  
Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the nine-month period ended September 30, 2024 and 2023 (in millions):

    

2024

    

2023

Balance as of January 1

$

612.5

$

585.3

Changes in estimates

 

(12.3)

 

Closure payments

 

 

(0.3)

Accretion expense

 

21.3

 

19.3

Balance as of September 30, 

$

621.6

$

604.3