FAIR VALUE - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Assets
Risk Management Assets	$ 1.8		$ 4.5
Liabilities
Risk Management Liabilities	43.3		49.1
Fair Value, Assets and Liabilities Transfers Between the Levels of the Fair Value Hierarchy [Abstract]
Fair Value Transfers, Amount	0	0
Fair value measurements on a recurring basis | Holding Company and Other | Level 1
Assets
Investment in Exchange-Traded Funds	112.3		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, Fair value
Assets
Investment in Exchange-Traded Funds	112.3		102.4
Fair value measurements on a recurring basis | Utility segments | Level 1
Assets
Risk Management Assets	0.1		0
Liabilities
Risk Management Liabilities	4.4		7.6
Fair value measurements on a recurring basis | Utility segments | Level 2
Assets
Risk Management Assets	1.0		2.3
Liabilities
Risk Management Liabilities	32.6		32.4
Fair value measurements on a recurring basis | Utility segments | Level 3
Assets
Risk Management Assets	0.7		2.2
Liabilities
Risk Management Liabilities	6.3		9.1
Fair value measurements on a recurring basis | Utility segments | Total, Fair value
Assets
Risk Management Assets	1.8		4.5
Liabilities
Risk Management Liabilities	43.3		49.1
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Level 1
Assets
Risk Management Assets	0.1		0
Liabilities
Risk Management Liabilities	2.3		4.8
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Level 2
Assets
Risk Management Assets	1.0		2.3
Liabilities
Risk Management Liabilities	31.4		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, Fair value
Assets
Risk Management Assets	1.1		2.3
Liabilities
Risk Management Liabilities	33.7		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	0.7		2.2
Liabilities
Risk Management Liabilities	0.1		0.3
Fair value measurements on a recurring basis | Utility segments | FTRs | Total, Fair value
Assets
Risk Management Assets	0.7		2.2
Liabilities
Risk Management Liabilities	0.1		0.3
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 1
Liabilities
Risk Management Liabilities	2.1		2.8
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 2
Liabilities
Risk Management Liabilities	0		0
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 3
Liabilities
Risk Management Liabilities	0		0
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Total, Fair value
Liabilities
Risk Management Liabilities	2.1		2.8
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Level 1
Liabilities
Risk Management Liabilities	0		0
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Level 2
Liabilities
Risk Management Liabilities	1.2		1.2
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Level 3
Assets
Risk Management Assets	0
Liabilities
Risk Management Liabilities	6.2		2.2
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Total, Fair value
Liabilities
Risk Management Liabilities	$ 7.4		$ 3.4