RISK MANAGEMENT ACTIVITIES - RISK MANAGEMENT ASSETS AND LIABILITIES (Details) $ in Millions	Jun. 30, 2015 USD ($) derivative	Dec. 31, 2014 USD ($)
Derivative Asset [Abstract]
Risk Management Assets	$ 7.7	$ 4.5
Derivative Liability [Abstract]
Risk Management Liabilities	$ 29.5	49.1
Designated as hedging instrument
Derivative, Number of Instruments Held [Abstract]
Number of derivative instruments held | derivative	0
Non-hedge derivatives
Derivative Asset [Abstract]
Other current assets from risk management activities	$ 6.7	4.0
Other long-term assets from risk management activities	1.0	0.5
Risk Management Assets	7.7	4.5
Derivative Liability [Abstract]
Other current liabilities from risk management activities	24.5	42.7
Other long-term liabilities from risk management activities	5.0	6.4
Risk Management Liabilities	29.5	49.1
Non-hedge derivatives | Natural gas contracts
Derivative Asset [Abstract]
Other current assets from risk management activities	2.6	1.8
Other long-term assets from risk management activities	1.0	0.5
Derivative Liability [Abstract]
Other current liabilities from risk management activities	19.5	37.3
Other long-term liabilities from risk management activities	2.9	5.3
Non-hedge derivatives | FTRs
Derivative Asset [Abstract]
Other current assets from risk management activities	4.1	2.2
Derivative Liability [Abstract]
Other current liabilities from risk management activities	0.0	0.3
Non-hedge derivatives | Petroleum product contracts
Derivative Asset [Abstract]
Other current assets from risk management activities	0.0	0.0
Other long-term assets from risk management activities		0.0
Derivative Liability [Abstract]
Other current liabilities from risk management activities	0.9	2.7
Other long-term liabilities from risk management activities		0.1
Non-hedge derivatives | Coal contracts
Derivative Asset [Abstract]
Other current assets from risk management activities	0.0	0.0
Other long-term assets from risk management activities	0.0	0.0
Derivative Liability [Abstract]
Other current liabilities from risk management activities	4.1	2.4
Other long-term liabilities from risk management activities	$ 2.1	$ 1.0