PENSION PLANS AND OTHER POST-RETIREMENT BENEFITS - Net Periodic Cost (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement charge	$ 0	$ (0.1)
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5.1	14.2	10.5
Interest cost	4.1	11.2	10.8
Expected long-term return on assets	(5.2)	(14.7)	(14.1)
Prior service cost amortization	0.1	0.5	0.6
Actuarial (gain) loss amortization	2.2	14.0	8.6
Settlement charge	10.1	1.4	0
Amortization of prior regulatory assets and liabilities	0.4	0	0
Net periodic expense	16.8	26.6	16.4
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.4	1.7	1.9
Interest cost	1.9	3.5	4.2
Expected long-term return on assets	(3.6)	(5.0)	(4.4)
Actuarial (gain) loss amortization	(1.0)	(0.1)	0
Transition obligation amortization	0	1.3	1.9
Amortization of prior regulatory assets and liabilities	(0.2)
Curtailment gain	0	(1.2)	0
Net periodic expense	$ (2.5)	$ 0.2	$ 3.6