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EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Components of net periodic benefit cost (credit)          
Contributions and payments related to pension and OPEB plans     $ 10.3 $ 11.0  
Total regulatory assets $ 3,241.0   3,241.0   $ 3,378.7
Pension Benefits          
Components of net periodic benefit cost (credit)          
Service cost 5.2 $ 6.1 15.7 18.2  
Interest cost 29.6 29.2 89.0 87.5  
Expected return on plan assets (43.8) (45.5) (131.5) (136.6)  
Loss (gain) on plan settlement (0.7) 0.7 (0.7) 0.7  
Amortization of prior service credit 0.0 (0.1) (0.1) (0.1)  
Amortization of net actuarial (gain) loss 9.0 14.9 32.1 44.6  
Net periodic benefit (credit) cost (0.7) 5.3 4.5 14.3  
Contributions and payments related to pension and OPEB plans     9.1    
Estimated future employer contributions for the remainder of the year 3.0   3.0    
Pension Benefits | Pension and Other Postretirement Plans Cost          
Components of net periodic benefit cost (credit)          
Total regulatory assets 5.8   5.8   24.9
Other Postretirement Benefits          
Components of net periodic benefit cost (credit)          
Service cost 2.9 2.7 8.5 8.1  
Interest cost 6.4 5.6 19.2 17.0  
Expected return on plan assets (13.6) (13.2) (40.7) (39.5)  
Amortization of prior service credit (1.4) (3.3) (6.4) (10.1)  
Amortization of net actuarial (gain) loss (2.2) (1.9) (5.9) (5.7)  
Net periodic benefit (credit) cost (7.9) $ (10.1) (25.3) $ (30.2)  
Contributions and payments related to pension and OPEB plans     1.2    
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)          
Total regulatory assets $ 23.3   $ 23.3   $ 38.2