EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Components of net periodic benefit cost (credit)
Regulatory assets	$ 432.2		$ 432.2		$ 449.7
Pension Benefits
Components of net periodic benefit cost (credit)
Service cost	0.9	$ 1.2	2.1	$ 2.5
Interest cost	7.7	7.3	15.3	14.7
Expected return on plan assets	(12.4)	(12.8)	(24.8)	(25.5)
Amortization of net actuarial loss	0.9	4.3	5.2	8.6
Net periodic benefit (credit) cost	(2.9)	0.0	(2.2)	0.3
Contributions and payments related to pension and OPEB plans			0.3
Estimated future employer contributions for the remainder of the year	0.2		0.2
Pension Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit)
Regulatory assets	8.1		8.1		12.9
Other Postretirement Benefits
Components of net periodic benefit cost (credit)
Service cost	0.8	0.7	1.5	1.5
Interest cost	1.9	1.7	3.7	3.3
Expected return on plan assets	(4.5)	(4.4)	(9.0)	(8.7)
Amortization of prior service credit	(1.1)	(2.6)	(3.6)	(5.1)
Amortization of net actuarial loss	0.2	0.7	0.8	1.2
Net periodic benefit (credit) cost	(2.7)	$ (3.9)	(6.6)	$ (7.8)
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 assets	$ 10.5		$ 10.5		$ 14.6