EMPLOYEE BENEFITS - Summary of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Net Regulatory Assets [Abstract]
Actuarial Losses/(Gains) Arising During the Year	$ 591.6	$ 48.6
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(137.8)	(140.1)
Unamortized Securitized Stranded Costs	0.0	(36.7)
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(0.7)	(3.9)
Prior Service Cost Securitized as Stranded Costs	0.0	(0.1)
Actuarial Loss	2,261.4	1,807.6
Prior Service Cost	5.6	6.3
Expected Actuarial Loss Expense	190.3
Expected Prior Service Cost Expense	0.9
OCI
Actuarial Losses/(Gains) Arising During the Year	15.4	0.7
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(5.4)	(5.6)
Actuarial Losses Securitized as Stranded Costs	0.0	0.0
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(0.2)	(0.4)
Prior Service Costs Securitized as Stranded Costs		0.0
Actuarial Loss	90.8	80.8
Prior Service Cost	0.9	1.1
Expected Actuarial Loss	7.2
Expected Prior Service Cost	0.2
Other Postretirement Benefits Plan
Net Regulatory Assets [Abstract]
Actuarial Losses/(Gains) Arising During the Year	4.6	6.4
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(8.0)	(9.9)
Unamortized Securitized Stranded Costs	0.0	(0.8)
Prior Service Cost Arising During the Year	0.0	1.3
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	25.1	23.6
Prior Service Cost Securitized as Stranded Costs	0.0	(1.3)
Actuarial Loss	203.9	207.3
Prior Service Cost	(172.5)	(197.6)
Expected Actuarial Loss Expense	8.3
Expected Prior Service Cost Expense	(21.4)
OCI
Actuarial Losses/(Gains) Arising During the Year	2.3	(1.2)
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(0.3)	(0.4)
Actuarial Losses Securitized as Stranded Costs	0.0	0.0
Prior Service Cost Arising During the Year	0.0	0.0
Prior Service (Cost)/Credit Reclassified as Net Periodic Benefit (Expense)/Income	(1.6)	0.0
Prior Service Costs Securitized as Stranded Costs		0.0
Actuarial Loss	7.0	5.0
Prior Service Cost	1.0	$ 2.6
Expected Actuarial Loss	0.2
Expected Prior Service Cost	$ 0.2