FAIR VALUE - SIGNIFICANT UNOBSERVABLE INPUTS (Details) (Utility segments, USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Significant internally-developed unobservable inputs
Risk Management Assets	4.5	$ 11.9
Risk Management Liabilities	49.1	4.1
Fair value measurements on a recurring basis \| Level 3
Significant internally-developed unobservable inputs
Risk Management Assets	2.2	1.7
Risk Management Liabilities	9.1	3.0
Fair value measurements on a recurring basis \| Level 3 \| Natural gas contracts
Significant internally-developed unobservable inputs
Risk Management Assets	0	0
Risk Management Liabilities	6.6	0
Fair value measurements on a recurring basis \| Level 3 \| Natural gas contracts \| Valuation Technique: Income-based \| Minimum
Significant internally-developed unobservable inputs
Option volatilities (as a percent)	50.50%
Fair value measurements on a recurring basis \| Level 3 \| Natural gas contracts \| Valuation Technique: Income-based \| Maximum
Significant internally-developed unobservable inputs
Option volatilities (as a percent)	67.20%
Fair value measurements on a recurring basis \| Level 3 \| FTRs
Significant internally-developed unobservable inputs
Risk Management Assets	2.2	1.5
Risk Management Liabilities	0.3	0.3
Fair value measurements on a recurring basis \| Level 3 \| FTRs \| Valuation Technique: Market-based \| Weighted Average
Significant internally-developed unobservable inputs
Forward market prices (in dollars per megawatt-month or ton)	188.16
Fair value measurements on a recurring basis \| Level 3 \| Coal contracts
Significant internally-developed unobservable inputs
Risk Management Assets	0	0.2
Risk Management Liabilities	2.2	$ 2.7
Fair value measurements on a recurring basis \| Level 3 \| Coal contracts \| Valuation Technique: Market-based \| Minimum
Significant internally-developed unobservable inputs
Forward market prices (in dollars per megawatt-month or ton)	10.89
Fair value measurements on a recurring basis \| Level 3 \| Coal contracts \| Valuation Technique: Market-based \| Maximum
Significant internally-developed unobservable inputs
Forward market prices (in dollars per megawatt-month or ton)	13.60

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given price.

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- Definition

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

+ References

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

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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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