EMPLOYEE BENEFITS - Summary of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Net Regulatory Assets [Abstract]
Actuarial Losses Arising During the Year	$ 553.1	$ 591.6
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(194.3)	(137.8)
Prior Service Cost Arising During the Year	2.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(1.0)	(0.7)
Actuarial Loss	2,620.2	2,261.4
Prior Service Cost	6.6	5.6
OCI
Actuarial Losses Arising During the Year	24.3	15.4
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(7.7)	(5.4)
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(0.2)	(0.2)
Actuarial Loss	107.4	90.8
Prior Service Cost	0.7	0.9
Other Postretirement Benefits Plan
Net Regulatory Assets [Abstract]
Actuarial Losses Arising During the Year	39.1	4.6
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(8.0)	(8.0)
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	21.3	25.1
Actuarial Loss	235.0	203.9
Prior Service Cost	(151.2)	(172.5)
OCI
Actuarial Losses Arising During the Year	1.3	2.3
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(0.4)	(0.3)
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(0.1)	(1.6)
Actuarial Loss	7.9	7.0
Prior Service Cost	$ 0.9	$ 1.0