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ASSET RETIREMENT OBLIGATION: (Tables)
12 Months Ended
Dec. 31, 2021
ASSET RETIREMENT OBLIGATION:  
Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the years ended December 31, 2021 and 2020 (in millions):

    

2021

    

2020

Balance as of January 1

$

545.0

$

262.3

Changes in estimates

 

 

269.3

Closure payments

 

(5.6)

 

(1.2)

Accretion expense

 

23.5

 

14.6

Balance as of December 31, 

$

562.9

$

545.0