EMPLOYEE BENEFITS - CHANGE IN BENEFIT OBLIGATION AND PLAN ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Change in benefit obligation
Obligation at January 1	$ 816.2	$ 734.0
Service cost	10.6	9.9	$ 8.7
Interest cost	21.9	25.2	28.2
Plan amendments	0.0	0.0
Actuarial loss (gain)	(37.1)	83.5
Participant contributions	0.0	0.0
Benefit payments	(38.5)	(36.4)
Obligation at December 31	773.1	816.2	734.0
Change in fair value of plan assets
Beginning balance at January 1	816.3	747.5
Actual return on plan assets	80.6	104.6
Employer contributions	1.0	0.6
Participant contributions	0.0	0.0
Benefit payments	(38.5)	(36.4)
Net transfer from affiliates	0.0	0.0
Ending balance at December 31	859.4	816.3	747.5
Funded status at December 31	86.3	0.1
OPEB Benefits
Change in benefit obligation
Obligation at January 1	148.5	147.2
Service cost	4.3	4.4	4.1
Interest cost	4.2	5.1	6.5
Plan amendments	(3.5)	0.0
Actuarial loss (gain)	2.3	(1.2)
Participant contributions	1.1	1.1
Benefit payments	(10.0)	(8.1)
Obligation at December 31	146.9	148.5	147.2
Change in fair value of plan assets
Beginning balance at January 1	288.3	263.4
Actual return on plan assets	23.3	31.1
Employer contributions	0.9	0.1
Participant contributions	1.1	1.1
Benefit payments	(10.0)	(8.1)
Net transfer from affiliates	0.0	0.7
Ending balance at December 31	303.6	288.3	$ 263.4
Funded status at December 31	$ 156.7	$ 139.8