RISK MANAGEMENT ACTIVITIES - UNREALIZED GAINS AND LOSSES (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Natural gas contracts | Income Statement - Operating and maintenance expense
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ (0.1)	$ 0.2
Natural gas contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	10.9	0.9
Natural gas contracts | Balance Sheet - Regulatory assets (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.6)	(0.2)
Natural gas contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(1.5)	3.4
Natural gas contracts | Balance Sheet - Regulatory liabilities (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.1	(0.4)
FTRs | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.2	0.1
FTRs | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.4)	(0.1)
Petroleum product contracts | Income Statement - Operating and maintenance expense
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.4	0
Petroleum product contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.4	0
Coal contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(2.6)	0.2
Coal contracts | Balance Sheet - Regulatory assets (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(2.0)	0.4
Coal contracts | Balance Sheet - Regulatory liabilities (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ 0	$ 1.6