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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