FAIR VALUE - SIGNIFICANT UNOBSERVABLE INPUTS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Significant internally-developed unobservable inputs
Risk Management Assets	$ 1.8	$ 4.5
Risk Management Liabilities	43.3	49.1
Fair value measurements on a recurring basis \| Utility segments \| Level 3
Significant internally-developed unobservable inputs
Risk Management Assets	0.7	2.2
Risk Management Liabilities	6.3	9.1
Fair value measurements on a recurring basis \| Utility segments \| Level 3 \| FTRs
Significant internally-developed unobservable inputs
Risk Management Assets	0.7	2.2
Risk Management Liabilities	0.1	0.3
Fair value measurements on a recurring basis \| Utility segments \| Level 3 \| FTRs \| Valuation Technique: Market-based \| Weighted Average
Significant internally-developed unobservable inputs
Forward market prices	159.42
Fair value measurements on a recurring basis \| Utility segments \| Level 3 \| Coal contracts
Significant internally-developed unobservable inputs
Risk Management Assets	0
Risk Management Liabilities	$ 6.2	$ 2.2
Fair value measurements on a recurring basis \| Utility segments \| Level 3 \| Coal contracts \| Valuation Technique: Market-based \| Minimum
Significant internally-developed unobservable inputs
Forward market prices	9.70
Fair value measurements on a recurring basis \| Utility segments \| Level 3 \| Coal contracts \| Valuation Technique: Market-based \| Maximum
Significant internally-developed unobservable inputs
Forward market prices	12.39

Represents the forward market prices, used as an input to measure the fair value.

No definition available.

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- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

+ Details

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- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

+ References

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