EMPLOYEE BENEFITS - Summary of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
OCI
Prior Service Costs Securitized as Stranded Costs		$ 0.0
Pension Plan
Net Regulatory Assets [Abstract]
Actuarial Losses Arising During the Year	$ 48.6	333.0
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(140.1)	(129.5)
Unamortized Securitized Stranded Costs	(36.7)	0.0
Prior Service Cost/(Credit) Arising During the Year	0.0	1.0
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(3.9)	(4.1)
Actuarial Loss	1,807.6	1,935.8
Prior Service Cost	6.3	10.3
Expected Actuarial Loss Expense	140.6
Expected Prior Service Cost Expense	0.9
OCI
Actuarial Losses Arising During the Year	0.7	9.3
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(5.6)	(5.7)
Prior Service Cost Reclassified as Net Periodic Benefit Expense	0.0	0.0
Prior Service (Credit)/Cost Arising During the Year	0.0	(0.4)
Prior Service Cost Reclassified as Net Periodic Benefit Expense	0.4	0.4
Prior Service Costs Securitized as Stranded Costs	(0.1)	0.0
Actuarial Loss	80.8	85.7
Prior Service Cost	1.1	1.5
Actuarial Loss	5.6
Prior Service Cost	0.2
Other Postretirement Benefits Plan
Net Regulatory Assets [Abstract]
Actuarial Losses Arising During the Year	6.4	44.8
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(9.9)	(8.6)
Prior Service Cost/(Credit) Arising During the Year	1.3	(4.0)
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(23.6)	(22.3)
Actuarial Loss	207.3	211.6
Prior Service Cost	(197.6)	(221.2)
Expected Actuarial Loss Expense	9.9
Expected Prior Service Cost Expense	(23.6)
OCI
Actuarial Losses Arising During the Year	1.2	(2.6)
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(0.4)	(0.5)
Prior Service Cost Reclassified as Net Periodic Benefit Expense	0.0	0.7
Prior Service (Credit)/Cost Arising During the Year	0.0	(0.1)
Prior Service Costs Securitized as Stranded Costs	(1.3)
Actuarial Loss	5.0	6.6
Prior Service Cost	2.6	2.6
Actuarial Loss	0.3
Prior Service Cost	0.2
Actuarial Gain (Loss) Securitized Stranded Costs, Regulatory Assets	(0.8)	0.0
Other Comprehensive Income, Defined Benefit Plan, Actuarial Losses Securitized Stranded Cost	$ 0.0	$ 0.0