Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012
Derivatives Fair Value [Line Items]
Derivative, Asset	$ 11.8	$ 19.2
Derivative, Liability	0.3	1.9
Gains (losses)	6.9	(28.4)
Other Deferred Charges and Other Assets [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative assets	0.4
Other Long-Term Liabilities [Member]
Derivatives Fair Value [Line Items]
Long-term portion of derivative liabilities	0
Natural Gas [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	5.6	3.0
Derivative, Liability	0.1	1.9
Volume	48.6 million Dth	77.2 million Dth
Gains (losses)	(8.5)	(36.3)
Fuel Oil [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	0.6	0.4
Derivative, Liability	0	0
Volume	8.6 million gallons	7.0 million gallons
Gains (losses)	0.5	1.8
FTRs [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	3.5	4.7
Derivative, Liability	0	0
Volume	25.3 million MWh	25.1 million MWh
Gains (losses)	14.9	6.1
Coal [Member]
Derivatives Fair Value [Line Items]
Derivative, Asset	2.1	11.1
Derivative, Liability	$ 0.2	$ 0

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.

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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

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 assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details

- Definition

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.

+ References+ Details