RISK MANAGEMENT ACTIVITIES - UNREALIZED GAINS AND LOSSES (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Natural gas contracts | Income Statement - Operating and maintenance expense
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ 0.2	$ (0.1)	$ 0.1	$ 0.1
Natural gas contracts | Balance Sheet - Regulatory assets
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	11.9	(1.0)	22.2	(0.3)
Natural gas contracts | Balance Sheet - Regulatory liabilities
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.8	(3.3)	(0.6)	(0.3)
FTRs | Balance Sheet - Regulatory assets
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	(7.2)	(1.0)	(7.0)	(0.9)
FTRs | Balance Sheet - Regulatory liabilities
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	2.4	1.1	2.0	1.0
Petroleum product contracts | Income Statement - Operating and maintenance expense
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.6	0.1	1.0	0.1
Petroleum product contracts | Balance Sheet - Regulatory assets
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.5	0.0	0.9	0.0
Coal contracts | Balance Sheet - Regulatory assets
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	0.6	(0.1)	(4.0)	0.5
Coal contracts | Balance Sheet - Regulatory liabilities
Risk management activities
Unrealized gain (loss) related to non-hedge derivative contracts	$ 0.0	$ 0.9	$ 0.0	$ 2.5