EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Components of net periodic benefit cost
Contributions and payments related to pension and OPEB plans	$ 4.1	$ 3.7
Pension Benefits
Components of net periodic benefit cost
Service cost	13.9	13.1
Interest cost	21.9	26.1
Expected return on plan assets	(50.6)	(47.9)
Loss on plan settlement	0.1	0.3
Amortization of prior service (credit) cost	0.4	0.4
Amortization of net actuarial (gain) loss	27.4	24.2
Net periodic benefit (credit) cost	13.1	16.2
Contributions and payments related to pension and OPEB plans	3.6
Estimated future employer contributions for the remainder of the year	7.9
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	4.2	4.1
Interest cost	3.6	4.7
Expected return on plan assets	(16.4)	(15.1)
Amortization of prior service (credit) cost	(4.0)	(3.7)
Amortization of net actuarial (gain) loss	(5.7)	(5.4)
Net periodic benefit (credit) cost	(18.3)	$ (15.4)
Contributions and payments related to pension and OPEB plans	0.5
Estimated future employer contributions for the remainder of the year	$ 1.6