EMPLOYEE BENEFITS-COSTS AND CONTRIBUTIONS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits
Components of net periodic benefit cost (credit)
Service cost	$ 3.9	$ 2.9	$ 7.2	$ 6.2
Interest cost	7.7	9.4	15.5	18.9
Expected return on plan assets	(17.4)	(17.3)	(35.1)	(34.7)
Amortization of prior service credit	(0.1)	0.0	(0.1)	0.0
Amortization of net actuarial (gain) loss	11.0	9.8	21.0	18.9
Net periodic benefit (credit) cost	5.1	4.8	8.5	9.3
Contributions and payments related to pension and OPEB plans			2.6
Estimated future employer contributions for the remainder of the year	1.0		1.0
Other Postretirement Benefits
Components of net periodic benefit cost (credit)
Service cost	0.9	1.0	2.1	2.1
Interest cost	1.4	1.7	2.7	3.4
Expected return on plan assets	(4.2)	(3.9)	(8.4)	(7.8)
Amortization of prior service credit	(0.3)	(0.2)	(0.6)	(0.3)
Amortization of net actuarial (gain) loss	(2.8)	(2.9)	(5.5)	(5.3)
Net periodic benefit (credit) cost	(5.0)	$ (4.3)	(9.7)	$ (7.9)
Estimated future employer contributions for the remainder of the year	$ 0.1		$ 0.1