RISK MANAGEMENT ACTIVITIES - UTILITY SEGMENTS, UNREALIZED GAINS AND LOSSES (Details) (Utility segments, 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 sales
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ 0	$ 0	$ 0.2
Natural gas contracts | Income Statement - Operating and maintenance expense
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.8)	0.1	0
Natural gas contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(38.0)	13.4	24.6
Natural gas contracts | Balance Sheet - Regulatory assets (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(5.2)	2.3	8.3
Natural gas contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(3.9)	4.6	(7.8)
Natural gas contracts | Balance Sheet - Regulatory liabilities (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.6)	0.3	0.3
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 | Income Statement - Operating and maintenance expense
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(1.7)	0.1	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 contracts | Balance Sheet - Regulatory assets (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(1.3)	(0.9)	(2.2)
Coal contracts | Balance Sheet - Regulatory assets (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0	3.5	0.1
Coal contracts | Balance Sheet - Regulatory liabilities (current)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(0.2)	(0.2)	0.3
Coal contracts | Balance Sheet - Regulatory liabilities (long-term)
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ (0.1)	$ (2.0)	$ 2.2