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)          
Regulatory assets $ 449.5   $ 449.5   $ 446.2
Regulatory liability 669.8   669.8   654.8
Pension Benefits          
Components of net periodic benefit cost (credit)          
Service cost 1.0 $ 0.9 2.1 $ 2.1  
Interest cost 7.6 7.7 15.2 15.3  
Expected return on plan assets (12.3) (12.4) (24.6) (24.8)  
Amortization of net actuarial loss 0.0 0.9 0.0 5.2  
Net periodic benefit credit (3.7) (2.9) (7.3) (2.2)  
Contributions and payments related to pension and OPEB plans     0.2    
Estimated future employer contributions for the remainder of the year 0.3   0.3    
Pension Benefits | Pension and Other Postretirement Plans Cost          
Components of net periodic benefit cost (credit)          
Regulatory liability 2.5   2.5   2.4
Other Postretirement Benefits          
Components of net periodic benefit cost (credit)          
Service cost 0.9 0.8 1.7 1.5  
Interest cost 2.0 1.9 4.0 3.7  
Expected return on plan assets (4.9) (4.5) (9.7) (9.0)  
Amortization of prior service credit (0.6) (1.1) (1.2) (3.6)  
Amortization of net actuarial loss 0.0 0.2 0.0 0.8  
Net periodic benefit credit (2.6) $ (2.7) (5.2) $ (6.6)  
Contributions and payments related to pension and OPEB plans     0.4    
Estimated future employer contributions for the remainder of the year 0.5   0.5    
Other Postretirement Benefits | Pension and Other Postretirement Plans Cost          
Components of net periodic benefit cost (credit)          
Regulatory liability         $ 0.3
Other Postretirement Benefits | Pension and Other Postretirement Plans Cost          
Components of net periodic benefit cost (credit)          
Regulatory assets $ 2.4   $ 2.4