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EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Components of net periodic benefit cost (credit)      
Contributions and payments related to pension and OPEB plans $ 3.9 $ 4.0  
Total regulatory assets 3,367.8   $ 3,378.7
Pension Benefits      
Components of net periodic benefit cost (credit)      
Service cost 5.7 6.7  
Interest cost 30.3 29.5  
Expected return on plan assets (44.1) (45.8)  
Amortization of net actuarial (gain) loss 13.6 14.4  
Net periodic benefit (credit) cost 5.5 4.8  
Contributions and payments related to pension and OPEB plans 3.5    
Estimated future employer contributions for the remainder of the year 8.6    
Pension Benefits | Pension and Other Postretirement Plans Cost      
Components of net periodic benefit cost (credit)      
Total regulatory assets 19.5   24.9
Other Postretirement Benefits      
Components of net periodic benefit cost (credit)      
Service cost 2.8 2.8  
Interest cost 6.5 5.7  
Expected return on plan assets (13.6) (13.2)  
Amortization of prior service credit (3.2) (3.4)  
Amortization of net actuarial (gain) loss (1.4) (1.9)  
Net periodic benefit (credit) cost (8.9) $ (10.0)  
Contributions and payments related to pension and OPEB plans 0.4    
Estimated future employer contributions for the remainder of the year 2.2    
Other Postretirement Benefits | Pension and Other Postretirement Plans Cost      
Components of net periodic benefit cost (credit)      
Total regulatory assets $ 33.5   $ 38.2