EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Components of net periodic benefit cost (credit)
Contributions and payments related to pension and OPEB plans	$ 3.6	$ 4.1
Pension Benefits
Components of net periodic benefit cost (credit)
Service cost	12.4	13.9
Interest cost	22.8	21.9
Expected return on plan assets	(52.7)	(50.6)
Loss on plan settlement	0.0	0.1
Amortization of prior service (credit) cost	0.4	0.4
Amortization of net actuarial (gain) loss	19.1	27.4
Net periodic benefit (credit) cost	2.0	13.1
Contributions and payments related to pension and OPEB plans	3.1
Estimated future employer contributions for the remainder of the year	8.0
Other Postretirement Benefits
Components of net periodic benefit cost (credit)
Service cost	3.8	4.2
Interest cost	3.9	3.6
Expected return on plan assets	(17.2)	(16.4)
Amortization of prior service (credit) cost	(4.0)	(4.0)
Amortization of net actuarial (gain) loss	(6.0)	(5.7)
Net periodic benefit (credit) cost	(19.5)	$ (18.3)
Contributions and payments related to pension and OPEB plans	0.5
Estimated future employer contributions for the remainder of the year	$ 2.1