EMPLOYEE BENEFITS - CHANGE IN BENEFIT OBLIGATION AND PLAN ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Change in benefit obligation
Obligation at January 1	$ 734.0	$ 664.1
Service cost	9.9	8.7	$ 10.4
Interest cost	25.2	28.2	26.0
Plan amendments	0.0	0.0
Net transfer from affiliates	0.0	3.6
Actuarial loss (gain)	83.5	64.4
Participant contributions	0.0	0.0
Benefit payments	(36.4)	(35.0)
Obligation at December 31	816.2	734.0	664.1
Change in fair value of plan assets
Beginning balance at January 1	747.5	639.3
Actual return on plan assets	104.6	139.0
Employer contributions	0.6	0.6
Participant contributions	0.0	0.0
Benefit payments	(36.4)	(35.0)
Net transfer from affiliates	0.0	3.6
Ending balance at December 31	816.3	747.5	639.3
Funded status at December 31	0.1	13.5
OPEB Benefits
Change in benefit obligation
Obligation at January 1	147.2	151.4
Service cost	4.4	4.1	6.1
Interest cost	5.1	6.5	8.2
Plan amendments	0.0	6.2
Net transfer from affiliates	0.0	0.0
Actuarial loss (gain)	(1.2)	(13.3)
Participant contributions	1.1	1.0
Benefit payments	(8.1)	(8.7)
Obligation at December 31	148.5	147.2	151.4
Change in fair value of plan assets
Beginning balance at January 1	263.4	231.7
Actual return on plan assets	31.1	38.6
Employer contributions	0.1	0.0
Participant contributions	1.1	1.0
Benefit payments	(8.1)	(8.7)
Net transfer from affiliates	0.7	0.8
Ending balance at December 31	288.3	263.4	$ 231.7
Funded status at December 31	$ 139.8	$ 116.2