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Equity Method Investments (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2010 | Dec. 31, 2011 | Dec. 31, 2011 Brazilian Transmission Lines [Member] | Dec. 31, 2010 Brazilian Transmission Lines [Member] | Nov. 30, 2011 Brazilian Transmission Lines [Member] | Nov. 30, 2010 Brazilian Transmission Lines [Member] | Dec. 31, 2009 Brazilian Transmission Lines [Member] | Nov. 30, 2010 ECTE [Member] | Nov. 30, 2011 ENTE [Member] | Nov. 30, 2011 ERTE [Member]
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Recognized gain on sale of ownership interests |  |  | $ 1.0 | $ 22.7 |  |  |  |  |  |
Recognized gain on sale of ownership interests, after tax | 13.8 |  | 0.6 | 13.8 |  |  |  |  |  |
Percentage Ownership Interest in Equity Method Investment Sold |  |  |  |  |  |  |  | 59.96% | 100.00% | 100.00%
Remaining Interest |  |  |  |  | 0.25 |  |  |  |  |
Number of separate parties with which company entered into agreements to sell equity ownership interests |  |  |  |  |  |  | 3 |  |  |
Number of years over which remaining ownership interest in equity method investment will be sold |  |  |  |  |  | 4 years |  |  |  |
Total assets of equity method investments | 107.4 | 111.1 |  |  |  |  |  |  |  |
Total long-term debt of equity method investments | 30.1 | 37.1 |  |  |  |  |  |  |  |
Investment in equity method investments | 10.9 | 9.2 |  |  |  |  |  |  |  |
Undistributed earnings of equity method investments | $ 1.9 | $ 3.7 |  |  |  |  |  |  |  |

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