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Investments (Unconsolidated Equity Investments) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net $ (11.4) $ (10.7)
WPL [Member]
   
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net (11.4) (10.8)
ATC [Member]
   
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net (11.2) (10.3)
ATC [Member] | WPL [Member]
   
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net (11.2) (10.3)
Other [Member]
   
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net (0.2) (0.4)
Other [Member] | WPL [Member]
   
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net (0.2) (0.5)
Totals [Member]
   
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net (11.4) (10.7)
Totals [Member] | WPL [Member]
   
Schedule of Equity Method Investments [Line Items]    
Equity (income) loss from unconsolidated investments, net $ (11.4) $ (10.8)