EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits
Components of net periodic benefit cost (credit)
Service cost	$ 3.6	$ 3.2	$ 10.8	$ 9.4
Interest cost	7.8	9.5	23.3	28.4
Expected return on plan assets	(17.6)	(17.4)	(52.7)	(52.1)
Loss on plan settlement	0.0	2.1	0.0	2.1
Amortization of prior service credit	0.0	(0.1)	(0.1)	(0.1)
Amortization of net actuarial (gain) loss	10.5	9.4	31.5	28.3
Net periodic benefit (credit) cost	4.3	6.7	12.8	16.0
Contributions and payments related to pension and OPEB plans			3.6
Other Postretirement Benefits
Components of net periodic benefit cost (credit)
Service cost	1.1	1.1	3.2	3.2
Interest cost	1.3	1.7	4.0	5.1
Expected return on plan assets	(4.2)	(4.0)	(12.6)	(11.8)
Amortization of prior service credit	(0.3)	(0.2)	(0.9)	(0.5)
Amortization of net actuarial (gain) loss	(2.7)	(2.6)	(8.2)	(7.9)
Net periodic benefit (credit) cost	$ (4.8)	$ (4.0)	$ (14.5)	$ (11.9)