RISK MANAGEMENT ACTIVITIES - UNREALIZED GAINS AND LOSSES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Natural gas contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ (0.3)	$ (0.2)	$ (0.4)	$ 0.3
Natural gas contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.5	0	0.4	(0.1)
FTRs | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.7	0.8	(0.2)	0
FTRs | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.3)	0	0.7	(0.3)
Petroleum product contracts | Operating and maintenance expense
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0	(0.1)	0	(0.1)
Petroleum product contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.4)	0.1	(0.4)	0
Petroleum product contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.1)	0	(0.1)	0
Coal contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.9)	(0.6)	(1.0)	2.1
Coal contracts | Balance Sheet - Regulatory assets (long Term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.1	0.2	0.7	4.2
Coal contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0	0	0	(0.3)
Coal contracts | Balance Sheet - Regulatory liabilities (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ (0.2)	$ 1.5	$ 2.3	$ (0.7)