v3.25.1
ASSET RETIREMENT OBLIGATION: (Tables)
3 Months Ended
Mar. 31, 2025
ASSET RETIREMENT OBLIGATION:  
Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the three-month periods ended March 31, 2025 and 2024 (in millions):

    

2025

    

2024

Balance as of January 1

$

546.1

$

612.5

Changes in estimates

 

(19.3)

 

(2.3)

Closure payments

 

 

Accretion expense

 

6.0

 

7.6

Balance as of March 31, 

$

532.9

$

617.8