EMPLOYEE BENEFITS - Summary of Changes in Plan Assets and Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Net Regulatory Assets [Abstract]
Actuarial Losses/(Gains) Arising During the Year	$ 184.6	$ (2.0)
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(119.9)	(142.3)
Prior Service Cost Arising During the Year	7.1	0.0
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(3.4)	(3.5)
Actuarial Loss	1,732.3	1,667.6
Expected Actuarial Loss Expense	128.5
Prior Service Cost	13.4	9.7
Expected Prior Service Cost Expense	4.1
OCI
Actuarial Losses/(Gains) Arising During the Year	6.8	(6.2)
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(5.8)	(6.2)
Prior Service (Credit)/Cost Arising During the Year	1.9	0.0
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(0.2)	(0.2)
Actuarial Loss	82.1	81.1
Expected Actuarial Loss Expense	5.8
Prior Service Cost	2.3	0.6
Expected Prior Service Cost Expense	0.2
Other Postretirement Benefit Plan [Member]
Net Regulatory Assets [Abstract]
Actuarial Losses/(Gains) Arising During the Year	32.4	(34.1)
Actuarial Losses Reclassified as Net Periodic Benefit Expense	(9.2)	(6.4)
Prior Service Cost Arising During the Year	(247.9)	0.0
Prior Service Cost Reclassified as Net Periodic Benefit Expense	9.7	0.5
Actuarial Loss	175.4	152.2
Expected Actuarial Loss Expense	7.9
Prior Service Cost	(239.5)	(1.3)
Expected Prior Service Cost Expense	(21.7)
OCI
Actuarial Losses/(Gains) Arising During the Year	(2.0)	0.7
Actuarial Losses Reclassified as Net Periodic Benefit Expense	0.2	(0.4)
Prior Service (Credit)/Cost Arising During the Year	4.0	0.0
Prior Service Cost Reclassified as Net Periodic Benefit Expense	(0.6)	0.0
Actuarial Loss	4.5	6.3
Expected Actuarial Loss Expense	0.4
Prior Service Cost	3.4	$ 0.0
Expected Prior Service Cost Expense	$ 0.2