EMPLOYEE BENEFITS - Summary of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Net Regulatory Assets [Abstract]
Actuarial Losses Arising During the Year	$ 333.0	$ 184.6
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(129.5)	(119.9)
Prior Service Cost/(Credit) Arising During the Year	1.0	7.1
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(4.1)	(3.4)
Actuarial Loss	1,935.8	1,732.3
Prior Service Cost	10.3	13.4
Expected Actuarial Loss Expense	141.8
Expected Prior Service Cost Expense	4.2
OCI
Actuarial Losses Arising During the Year	9.3	6.8
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(5.7)	(5.8)
Prior Service (Credit)/Cost Arising During the Year	(0.4)	1.9
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(0.4)	(0.2)
Actuarial Loss	85.7	82.1
Prior Service Cost	1.5	2.3
Actuarial Loss	5.8
Prior Service Cost	0.3
Other Postretirement Benefits Plan
Net Regulatory Assets [Abstract]
Actuarial Losses Arising During the Year	44.8	32.4
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(8.6)	(9.2)
Prior Service Cost/(Credit) Arising During the Year	(4.0)	(247.9)
Prior Service Cost Reclassified as Net Periodic Benefit Expense	22.3	9.7
Actuarial Loss	211.6	175.4
Prior Service Cost	(221.2)	(239.5)
Expected Actuarial Loss Expense	8.8
Expected Prior Service Cost Expense	(21.7)
OCI
Actuarial Losses Arising During the Year	2.6	(2.0)
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(0.5)	0.2
Prior Service (Credit)/Cost Arising During the Year	(0.1)	4.0
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(0.7)	(0.6)
Actuarial Loss	6.6	4.5
Prior Service Cost	2.6	$ 3.4
Actuarial Loss	0.3
Prior Service Cost	$ 0.2