v3.26.1
ASSET RETIREMENT OBLIGATION: (Tables)
6 Months Ended
Jun. 30, 2026
ASSET RETIREMENT OBLIGATION:  
Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the six-month periods ended June 30, 2026 and 2025 (in millions):

  ​ ​ ​

2026

  ​ ​ ​

2025

Balance as of January 1

$

471.1

$

546.1

Changes in estimates

 

9.2

 

(48.5)

Closure payments

 

(2.4)

 

Accretion expense

 

12.8

 

12.2

Balance as of June 30, 

$

490.7

$

509.9