FAIR VALUE - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets
Risk Management Assets	$ 330.4	$ 315.5
Liabilities
Risk Management Liabilities	222.0	226.6
Fair value measurements on a recurring basis | Level 1
Assets
Risk Management Assets	109.6	83.9
Liabilities
Risk Management Liabilities	122.7	113.6
Fair value measurements on a recurring basis | Level 2
Assets
Risk Management Assets	168.8	177.8
Liabilities
Risk Management Liabilities	72.5	81.3
Fair value measurements on a recurring basis | Level 3
Assets
Risk Management Assets	52.0	53.8
Liabilities
Risk Management Liabilities	26.8	31.7
Fair value measurements on a recurring basis | Total, Fair value
Assets
Risk Management Assets	330.4	315.5
Liabilities
Risk Management Liabilities	222.0	226.6
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Level 1
Assets
Risk Management Assets	0.9	2.4
Liabilities
Risk Management Liabilities	0.2	0.5
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Level 2
Assets
Risk Management Assets	12.4	7.7
Liabilities
Risk Management Liabilities	0.2	0.6
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	0
Fair value measurements on a recurring basis | Utility segments | Natural gas contracts | Total, Fair value
Assets
Risk Management Assets	13.3	10.1
Liabilities
Risk Management Liabilities	0.4	1.1
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	0.9	2.1
Liabilities
Risk Management Liabilities	0.2	0.3
Fair value measurements on a recurring basis | Utility segments | FTRs | Total, Fair value
Assets
Risk Management Assets	0.9	2.1
Liabilities
Risk Management Liabilities	0.2	0.3
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 1
Assets
Risk Management Assets	0.1	0.1
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 2
Assets
Risk Management Assets	0	0
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Level 3
Assets
Risk Management Assets	0	0
Fair value measurements on a recurring basis | Utility segments | Petroleum product contracts | Total, Fair value
Assets
Risk Management Assets	0.1	0.1
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	0
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Level 3
Assets
Risk Management Assets	1.8	0.2
Liabilities
Risk Management Liabilities	1.5	2.7
Fair value measurements on a recurring basis | Utility segments | Coal contracts | Total, Fair value
Assets
Risk Management Assets	1.8	0.2
Liabilities
Risk Management Liabilities	1.5	2.7
Fair value measurements on a recurring basis | Nonregulated segments | Level 1
Assets
Investment in Exchange-Traded Funds	16.2	15.9
Liabilities
Contingent consideration related to the acquisition of Compass	0	0
Fair value measurements on a recurring basis | Nonregulated segments | Level 2
Assets
Investment in Exchange-Traded Funds	0	0
Liabilities
Contingent consideration related to the acquisition of Compass	0	0
Fair value measurements on a recurring basis | Nonregulated segments | Level 3
Assets
Investment in Exchange-Traded Funds	0	0
Liabilities
Contingent consideration related to the acquisition of Compass	7.8	7.8
Fair value measurements on a recurring basis | Nonregulated segments | Total, Fair value
Assets
Investment in Exchange-Traded Funds	16.2	15.9
Liabilities
Contingent consideration related to the acquisition of Compass	7.8	7.8
Fair value measurements on a recurring basis | Nonregulated segments | Natural gas contracts | Level 1
Assets
Risk Management Assets	20.6	16.3
Liabilities
Risk Management Liabilities	10.2	14.3
Fair value measurements on a recurring basis | Nonregulated segments | Natural gas contracts | Level 2
Assets
Risk Management Assets	28.8	35.2
Liabilities
Risk Management Liabilities	20.4	22.0
Fair value measurements on a recurring basis | Nonregulated segments | Natural gas contracts | Level 3
Assets
Risk Management Assets	27.5	35.6
Liabilities
Risk Management Liabilities	22.0	25.2
Fair value measurements on a recurring basis | Nonregulated segments | Natural gas contracts | Total, Fair value
Assets
Risk Management Assets	76.9	87.1
Liabilities
Risk Management Liabilities	52.6	61.5
Fair value measurements on a recurring basis | Nonregulated segments | Electric contracts | Level 1
Assets
Risk Management Assets	88.0	65.1
Liabilities
Risk Management Liabilities	112.3	98.8
Fair value measurements on a recurring basis | Nonregulated segments | Electric contracts | Level 2
Assets
Risk Management Assets	127.6	134.9
Liabilities
Risk Management Liabilities	51.9	58.7
Fair value measurements on a recurring basis | Nonregulated segments | Electric contracts | Level 3
Assets
Risk Management Assets	21.8	15.9
Liabilities
Risk Management Liabilities	3.1	3.5
Fair value measurements on a recurring basis | Nonregulated segments | Electric contracts | Total, Fair value
Assets
Risk Management Assets	237.4	215.9
Liabilities
Risk Management Liabilities	$ 167.3	$ 161.0