FAIR VALUE - SIGNIFICANT UNOBSERVABLE INPUTS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Risk Management Asset	$ 2.3	$ 2.4
Risk Management Liability	7.0	3.1
Fair value measurements on a recurring basis \| Level 3
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Risk Management Asset	2.2	1.7
Risk Management Liability	2.5	3.0
Fair value measurements on a recurring basis \| Level 3 \| FTRs
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Risk Management Asset	2.2	1.5
Risk Management Liability	0.3	0.3
Fair value measurements on a recurring basis \| Level 3 \| FTRs \| Valuation Technique: Market-based \| Average
Fair Value Inputs
Forward market prices (in dollars per megawatt-month or ton)	188.16
Fair value measurements on a recurring basis \| Level 3 \| Coal contract
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Risk Management Asset	0	0.2
Risk Management Liability	$ 2.2	$ 2.7
Fair value measurements on a recurring basis \| Level 3 \| Coal contract \| Valuation Technique: Market-based \| Minimum
Fair Value Inputs
Forward market prices (in dollars per megawatt-month or ton)	10.89
Fair value measurements on a recurring basis \| Level 3 \| Coal contract \| Valuation Technique: Market-based \| Maximum
Fair Value Inputs
Forward market prices (in dollars per megawatt-month or ton)	13.60

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.

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.

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- Definition

The forward market price on the future contracts used as an input to measure fair value.

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