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ASSET RETIREMENT OBLIGATION: (Tables)
9 Months Ended
Sep. 30, 2022
ASSET RETIREMENT OBLIGATION:  
Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the first nine months of 2022 and 2021 (in millions):

    

2022

    

2021

Balance as of January 1

$

562.9

$

545.0

Changes in estimates

 

43.3

 

Closure payments

 

(4.9)

 

(3.7)

Accretion expense

 

19.5

 

18.3

Balance as of September 30, 

$

620.8

$

559.6