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Investments (Unconsolidated Equity Investments) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net $ (10.9) $ (10.6) $ (21.6) $ (20.0)
WPL [Member]
        
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net (10.8) (10.6) (21.6) (20.7)
American Transmission Company LLC (ATC) [Member]
        
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net (10.6) (10.3) (20.9) (20.2)
American Transmission Company LLC (ATC) [Member] | WPL [Member]
        
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net (10.6) (10.3) (20.9) (20.2)
Other [Member]
        
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net (0.3) (0.3) (0.7) 0.2
Other [Member] | WPL [Member]
        
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net (0.2) (0.3) (0.7) (0.5)
Totals [Member]
        
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net (10.9) (10.6) (21.6) (20.0)
Totals [Member] | WPL [Member]
        
Schedule of Equity Method Investments [Line Items]        
Equity income from unconsolidated investments, net $ (10.8) $ (10.6) $ (21.6) $ (20.7)