PENSION PLANS AND OTHER POST-RETIREMENT BENEFITS - Expenses Recognized in Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial (gain) loss experienced during the year	$ 1.5	$ (14.1)	$ 28.7
Net actuarial gain (loss) recognized as expense	0	(8.9)	(4.9)
Prior service cost recognized as expense	0	(0.3)	(0.3)
Total recognized in other comprehensive income (loss)	1.5	(23.3)	23.5
Transfer to regulatory assets (liabilities)	(1.5)	0	0
Other Comprehensive Income	0	(23.3)	23.5
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial (gain) loss experienced during the year	1.1	(8.1)	(1.8)
Net actuarial gain (loss) recognized as expense	0	0.6	0
Transition obligation recognized as expense	0	(0.3)	(0.3)
Total recognized in other comprehensive income (loss)	1.1	(7.8)	(2.1)
Transfer to regulatory assets (liabilities)	(1.1)	0	0
Other Comprehensive Income	$ 0	$ (7.8)	$ (2.1)