v3.26.1
EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Components of net periodic benefit cost (credit)          
Contributions and payments related to pension and OPEB plans     $ 7.2 $ 7.1  
Regulatory assets $ 3,169.1   3,169.1   $ 3,173.6
Regulatory liabilities 4,325.5   4,325.5   4,210.2
Pension Benefits          
Components of net periodic benefit cost (credit)          
Service cost 5.1 $ 4.8 11.2 10.5  
Interest cost 28.6 29.1 57.2 59.4  
Expected return on plan assets (42.6) (43.6) (85.4) (87.7)  
Amortization of prior service credit (0.1) (0.1) (0.1) (0.1)  
Amortization of net actuarial (gain) loss 10.9 9.5 21.3 23.1  
Net periodic benefit (credit) cost 1.9 (0.3) 4.2 5.2  
Contributions and payments related to pension and OPEB plans     6.1    
Estimated future employer contributions for the remainder of the year 5.7   5.7    
Pension Benefits | Pension and Other Postretirement Plans Cost          
Components of net periodic benefit cost (credit)          
Regulatory liabilities 19.3   19.3   14.7
Other Postretirement Benefits          
Components of net periodic benefit cost (credit)          
Service cost 3.1 2.8 6.2 5.6  
Interest cost 6.5 6.3 13.2 12.8  
Expected return on plan assets (14.5) (13.5) (28.9) (27.1)  
Amortization of prior service credit (1.5) (1.8) (2.6) (5.0)  
Amortization of net actuarial (gain) loss (2.4) (2.3) (4.1) (3.7)  
Net periodic benefit (credit) cost (8.8) $ (8.5) (16.2) $ (17.4)  
Contributions and payments related to pension and OPEB plans     1.1    
Estimated future employer contributions for the remainder of the year 1.2   1.2    
Other Postretirement Benefits | Pension and Other Postretirement Plans Cost          
Components of net periodic benefit cost (credit)          
Regulatory assets $ 7.9   $ 7.9   $ 0.7