RISK MANAGEMENT ACTIVITIES - UNREALIZED GAINS AND LOSSES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Natural gas contracts | Income Statement - Cost of fuel, natural gas and purchased power
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ 0	$ 0	$ 0.2
Natural gas contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(2.3)	0.7	2.2
Natural gas contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.3)	0.3	0.1
FTRs | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0	0.2	(0.1)
FTRs | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.4	(0.3)	0
Petroleum product contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(1.1)	0	0.1
Petroleum product contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.1)	0.1	0
Coal contract | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(1.3)	(0.9)	(2.2)
Coal contract | Balance Sheet - Regulatory assets (long Term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.2)	3.5	0.1
Coal contract | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0	(0.2)	0.3
Coal contract | Balance Sheet - Regulatory liabilities (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ (0.1)	$ (2.0)	$ 2.2