FAIR VALUE (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets
Risk Management Assets	$ 232.8	$ 190.7
Liabilities
Risk Management Liabilities	190.2	240.3
Level 1 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	75.5	70.3
Investment in exchange-traded funds	12.4	11.8
Liabilities
Risk Management Liabilities	87.6	73.7
Level 2 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	139.6	100.8
Liabilities
Risk Management Liabilities	89.0	142.1
Level 3 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	17.7	19.6
Liabilities
Risk Management Liabilities	13.6	24.5
Total, Fair value | Fair value measurements on a recurring basis
Assets
Risk Management Assets	232.8	190.7
Investment in exchange-traded funds	12.4	11.8
Liabilities
Risk Management Liabilities	190.2	240.3
Utility Segments | Natural gas contracts | Level 1 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	1.2	0.3
Liabilities
Risk Management Liabilities		1.1
Utility Segments | Natural gas contracts | Level 2 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	10.5	3.1
Liabilities
Risk Management Liabilities	3.8	14.1
Utility Segments | Natural gas contracts | Total, Fair value | Fair value measurements on a recurring basis
Assets
Risk Management Assets	11.7	3.4
Liabilities
Risk Management Liabilities	3.8	15.2
Utility Segments | FTRs | Level 3 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	1.0	2.1
Liabilities
Risk Management Liabilities	0.1	0.1
Utility Segments | FTRs | Total, Fair value | Fair value measurements on a recurring basis
Assets
Risk Management Assets	1.0	2.1
Liabilities
Risk Management Liabilities	0.1	0.1
Utility Segments | Petroleum product contracts | Level 1 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	0.1	0.2
Utility Segments | Petroleum product contracts | Total, Fair value | Fair value measurements on a recurring basis
Assets
Risk Management Assets	0.1	0.2
Utility Segments | Coal contract | Level 2 | Fair value measurements on a recurring basis
Liabilities
Risk Management Liabilities	0.1
Utility Segments | Coal contract | Level 3 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	0.1	2.5
Liabilities
Risk Management Liabilities	4.7	9.0
Utility Segments | Coal contract | Total, Fair value | Fair value measurements on a recurring basis
Assets
Risk Management Assets	0.1	2.5
Liabilities
Risk Management Liabilities	4.8	9.0
Nonregulated Segments | Natural gas contracts | Level 1 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	10.2	21.4
Liabilities
Risk Management Liabilities	9.0	17.7
Nonregulated Segments | Natural gas contracts | Level 2 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	21.2	36.4
Liabilities
Risk Management Liabilities	14.8	36.9
Nonregulated Segments | Natural gas contracts | Level 3 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	2.9	5.4
Liabilities
Risk Management Liabilities	1.2	1.5
Nonregulated Segments | Natural gas contracts | Total, Fair value | Fair value measurements on a recurring basis
Assets
Risk Management Assets	34.3	63.2
Liabilities
Risk Management Liabilities	25.0	56.1
Nonregulated Segments | Electric contracts | Level 1 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	64.0	48.4
Liabilities
Risk Management Liabilities	78.6	54.9
Nonregulated Segments | Electric contracts | Level 2 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	107.9	61.3
Liabilities
Risk Management Liabilities	70.3	91.1
Nonregulated Segments | Electric contracts | Level 3 | Fair value measurements on a recurring basis
Assets
Risk Management Assets	13.7	9.6
Liabilities
Risk Management Liabilities	7.6	13.9
Nonregulated Segments | Electric contracts | Total, Fair value | Fair value measurements on a recurring basis
Assets
Risk Management Assets	185.6	119.3
Liabilities
Risk Management Liabilities	$ 156.5	$ 159.9