Related Parties (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Related Party Transaction [Line Items]
Income (Loss) from Equity Method Investments, Net of Dividends or Distributions	$ 62.5	$ 60.1	$ 59.1
Distributions from transmission affiliate	49.7	49.3	46.6
American Transmission Company [Member]
Provided and received services from the associated companies
Services Received, -ATC	219.2	220.8	196.0
Receivable and payable balances with ATC
Services Provided, -ATC	5.4	3.8
Services Received, -ATC	18.1	18.5
Related Parties (Textuals) [Abstract]
Equity interest in ATC	26.20%
Services [Member] \| American Transmission Company [Member]
Provided and received services from the associated companies
Services Provided, -ATC	10.8	16.9	22.3
Receivable and payable balances with ATC
Services Provided, -ATC	$ 0.7	$ 0.9

For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

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-Publisher FASB

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Expenses recognized during the period resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

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