FAIR VALUE - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014
Utility segments
Assets
Risk Management Assets	$ 11.9	$ 4.5
Liabilities
Risk Management Liabilities	4.1	49.1
Fair value measurements on a recurring basis | Utility segments
Transfers Between the Levels of the Fair Value Hierarchy
Fair Value Transfers, Amount	0
Fair value measurements on a recurring basis | Utility segments | Level 1
Assets
Risk Management Assets	2.5	0
Liabilities
Risk Management Liabilities	0.5	7.6
Fair value measurements on a recurring basis | Utility segments | Level 2
Assets
Risk Management Assets	7.7	2.3
Liabilities
Risk Management Liabilities	0.6	32.4
Fair value measurements on a recurring basis | Utility segments | Level 3
Assets
Risk Management Assets	1.7	2.2
Liabilities
Risk Management Liabilities	3.0	9.1
Fair value measurements on a recurring basis | Utility segments | Total
Assets
Risk Management Assets	11.9	4.5
Liabilities
Risk Management Liabilities	4.1	49.1
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Level 1
Assets
Risk Management Assets	2.4	0
Liabilities
Risk Management Liabilities	0.5	4.8
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Level 2
Assets
Risk Management Assets	7.7	2.3
Liabilities
Risk Management Liabilities	0.6	31.2
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Level 3
Assets
Risk Management Assets	0	0
Liabilities
Risk Management Liabilities	0	6.6
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Total
Assets
Risk Management Assets	10.1	2.3
Liabilities
Risk Management Liabilities	1.1	42.6
Fair value measurements on a recurring basis | Utility segments | FTRs | Level 1
Assets
Risk Management Assets	0	0
Liabilities
Risk Management Liabilities	0	0
Fair value measurements on a recurring basis | Utility segments | FTRs | Level 2
Assets
Risk Management Assets	0	0
Liabilities
Risk Management Liabilities	0	0
Fair value measurements on a recurring basis | Utility segments | FTRs | Level 3
Assets
Risk Management Assets	1.5	2.2
Liabilities
Risk Management Liabilities	0.3	0.3
Fair value measurements on a recurring basis | Utility segments | FTRs | Total
Assets
Risk Management Assets	1.5	2.2
Liabilities
Risk Management Liabilities	0.3	0.3
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 1
Assets
Risk Management Assets	0.1
Liabilities
Risk Management Liabilities		2.8
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 2
Assets
Risk Management Assets	0
Liabilities
Risk Management Liabilities		0
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 3
Assets
Risk Management Assets	0
Liabilities
Risk Management Liabilities		0
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Total
Assets
Risk Management Assets	0.1
Liabilities
Risk Management Liabilities		2.8
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Level 1
Assets
Risk Management Assets	0	0
Liabilities
Risk Management Liabilities	0	0
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Level 2
Assets
Risk Management Assets	0	0
Liabilities
Risk Management Liabilities	0	1.2
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Level 3
Assets
Risk Management Assets	0.2	0
Liabilities
Risk Management Liabilities	2.7	2.2
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Total
Assets
Risk Management Assets	0.2	0
Liabilities
Risk Management Liabilities	2.7	3.4
Fair value measurements on a recurring basis | Holding Company and Other | Level 1
Assets
Investment in exchange-traded funds	15.9	102.4
Fair value measurements on a recurring basis | Holding Company and Other | Level 2
Assets
Investment in exchange-traded funds	0	0
Fair value measurements on a recurring basis | Holding Company and Other | Level 3
Assets
Investment in exchange-traded funds	0	0
Fair value measurements on a recurring basis | Holding Company and Other | Total
Assets
Investment in exchange-traded funds	$ 15.9	$ 102.4