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EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Dec. 31, 2022
Components of net periodic benefit cost (credit)      
Contributions and payments related to pension and OPEB plans $ 5.5 $ 3.6  
Total regulatory assets 3,325.3   $ 3,306.9
Pension and OPEB costs      
Components of net periodic benefit cost (credit)      
Total regulatory assets 701.9   $ 714.3
Pension Benefits      
Components of net periodic benefit cost (credit)      
Service cost 6.6 12.4  
Interest cost 30.8 22.8  
Expected return on plan assets (47.4) (52.7)  
Amortization of prior service (credit) cost 0.1 0.4  
Amortization of net actuarial (gain) loss 7.4 19.1  
Net periodic benefit (credit) cost (2.5) 2.0  
Contributions and payments related to pension and OPEB plans 4.9    
Estimated future employer contributions for the remainder of the year 9.6    
Pension Benefits | Pension and Other Postretirement Plans Cost      
Components of net periodic benefit cost (credit)      
Total regulatory assets 2.1    
Other Postretirement Benefits      
Components of net periodic benefit cost (credit)      
Service cost 2.5 3.8  
Interest cost 5.4 3.9  
Expected return on plan assets (13.3) (17.2)  
Amortization of prior service (credit) cost (3.7) (4.0)  
Amortization of net actuarial (gain) loss (3.2) (6.0)  
Net periodic benefit (credit) cost (12.3) $ (19.5)  
Contributions and payments related to pension and OPEB plans 0.6    
Estimated future employer contributions for the remainder of the year 1.5    
Other Postretirement Benefits | Pension and Other Postretirement Plans Cost      
Components of net periodic benefit cost (credit)      
Total regulatory assets $ 3.7