RISK MANAGEMENT ACTIVITIES - UNREALIZED GAINS AND LOSSES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Natural gas contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ (0.3)	$ (0.5)	$ (0.1)	$ 0.5
Natural gas contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.2)	(0.9)	(0.1)	(0.1)
FTRs | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(1.0)	(1.0)	(0.9)	(0.8)
FTRs | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	1.1	0.1	1.0	(0.3)
Petroleum product contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0	(0.1)	0	(0.1)
Coal contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.3)	0.8	(0.1)	2.7
Coal contracts | Balance Sheet - Regulatory assets (long Term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.2	1.7	0.6	4.0
Coal contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0	(0.1)	0	(0.3)
Coal contracts | Balance Sheet - Regulatory liabilities (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ 0.9	$ 0	$ 2.5	$ (2.2)