Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Derivatives Fair Value [Line Items]
Derivative, Asset	$ 15.3us-gaap_DerivativeAssets	$ 11.8us-gaap_DerivativeAssets
Derivative, Liability	12.5us-gaap_DerivativeLiabilities	0.3us-gaap_DerivativeLiabilities
Gains (losses)	20.5	6.9
Other Deferred Charges and Other Assets [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative assets	0.6
Other Long-Term Liabilities [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative liabilities	0.8
Natural Gas [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	5.0	5.6
Derivative, Liability	12.3	0.1
Volume	40.5 million Dth	48.6 million Dth
Gains (losses)	7.3	(8.5)
Fuel Oil [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	0	0.6
Derivative, Liability	0	0
Volume	9.2 million gallons	8.6 million gallons
Gains (losses)	0.5	0.5
FTRs [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	7.0	3.5
Derivative, Liability	0	0
Volume	26.1 million MWh	25.3 million MWh
Gains (losses)	12.7	14.9
Coal [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	3.3	2.1
Derivative, Liability	$ 0.2	$ 0.2

Fair value, after the 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 not elected to be offset.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

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- Definition

Fair value, after the 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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

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

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Fair value, after the 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 not elected to be offset.

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Fair value, after the 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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Volume Of Derivative Instruments.

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