EMPLOYEE BENEFITS - Summary of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Pension Plan
Net Regulatory Assets [Abstract]
Actuarial (Gains)/Losses Arising During the Year	$ (961.7)	$ 553.1
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(231.2)	(194.3)
Prior Service Cost Arising During the Year	0.0	2.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(1.3)	(1.0)
Actuarial Loss	1,427.3	2,620.2
Prior Service Cost	5.3	6.6
OCI
Actuarial (Gains)/Losses Arising During the Year	(28.4)	24.3
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(12.7)	(7.7)
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(0.1)	(0.2)
Actuarial Loss	66.3	107.4
Prior Service Cost	0.6	0.7
Other Postretirement Benefits Plan
Net Regulatory Assets [Abstract]
Actuarial (Gains)/Losses Arising During the Year	(181.5)	39.1
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(8.5)	(8.0)
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	21.1	21.3
Actuarial Loss	45.0	235.0
Prior Service Cost	(130.1)	(151.2)
OCI
Actuarial (Gains)/Losses Arising During the Year	(4.0)	1.3
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(0.4)	(0.4)
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	0.1	(0.1)
Actuarial Loss	3.5	7.9
Prior Service Cost	$ 1.0	$ 0.9