EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Components of net periodic benefit cost (credit)
Contributions and payments related to pension and OPEB plans	$ 4.0	$ 5.5
Total regulatory assets	3,286.9		$ 3,274.7
Pension Benefits
Components of net periodic benefit cost (credit)
Service cost	6.7	6.6
Interest cost	29.5	30.8
Expected return on plan assets	(45.8)	(47.4)
Amortization of prior service (credit) cost	0.0	0.1
Amortization of net actuarial (gain) loss	14.4	7.4
Net periodic benefit (credit) cost	4.8	(2.5)
Contributions and payments related to pension and OPEB plans	3.7
Estimated future employer contributions for the remainder of the year	9.5
Pension Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit)
Total regulatory assets	10.8
Other Postretirement Benefits
Components of net periodic benefit cost (credit)
Service cost	2.8	2.5
Interest cost	5.7	5.4
Expected return on plan assets	(13.2)	(13.3)
Amortization of prior service (credit) cost	(3.4)	(3.7)
Amortization of net actuarial (gain) loss	(1.9)	(3.2)
Net periodic benefit (credit) cost	(10.0)	$ (12.3)
Contributions and payments related to pension and OPEB plans	0.3
Estimated future employer contributions for the remainder of the year	1.9
Other Postretirement Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit)
Total regulatory assets	$ 20.7