EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Components of net periodic benefit cost (credit)
Contributions and payments related to pension and OPEB plans			$ 13.0	$ 11.6
Total regulatory assets	$ 3,212.9		3,212.9		$ 3,306.9
Pension Benefits
Components of net periodic benefit cost (credit)
Service cost	6.0	$ 12.3	18.0	38.9
Interest cost	30.5	23.2	91.7	68.4
Expected return on plan assets	(46.8)	(51.6)	(140.6)	(156.8)
Loss on plan settlement	0.0	1.1	0.0	3.3
Amortization of prior service (credit) cost	0.0	0.4	0.1	1.2
Amortization of net actuarial (gain) loss	8.1	19.0	24.8	57.2
Net periodic benefit (credit) cost	(2.2)	4.4	(6.0)	12.2
Contributions and payments related to pension and OPEB plans			11.4
Estimated future employer contributions for the remainder of the year	3.2		3.2
Pension Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit)
Total regulatory assets	8.0		8.0
Other Postretirement Benefits
Components of net periodic benefit cost (credit)
Service cost	2.4	3.6	7.3	10.7
Interest cost	5.4	3.8	16.2	11.5
Expected return on plan assets	(13.3)	(17.2)	(39.8)	(51.7)
Amortization of prior service (credit) cost	(3.7)	(4.0)	(11.1)	(11.9)
Amortization of net actuarial (gain) loss	(3.0)	(6.2)	(9.2)	(18.5)
Net periodic benefit (credit) cost	(12.2)	$ (20.0)	(36.6)	$ (59.9)
Contributions and payments related to pension and OPEB plans			1.6
Estimated future employer contributions for the remainder of the year	0.3		0.3
Other Postretirement Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit)
Total regulatory assets	$ 11.1		$ 11.1