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Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Production Tax Credits [Line Items]    
Production Tax Credits $ 10.0 $ 8.6
WPL [Member]
    
Production Tax Credits [Line Items]    
Production Tax Credits 5.4 4.7
Cedar Ridge Wind Project [Member] | WPL [Member]
    
Production Tax Credits [Line Items]    
Nameplate Capacity (in megawatts) 68  
Bent Tree Phase I Wind Project [Member] | WPL [Member]
    
Production Tax Credits [Line Items]    
Nameplate Capacity (in megawatts) 201  
Whispering Willow East Wind Project [Member] | IPL [Member]
    
Production Tax Credits [Line Items]    
Nameplate Capacity (in megawatts) 200  
Bent Tree Phase I Wind Project [Member] | WPL [Member]
    
Production Tax Credits [Line Items]    
Production Tax Credits 4.2 3.5
Cedar Ridge Wind Project [Member] | WPL [Member]
    
Production Tax Credits [Line Items]    
Production Tax Credits 1.2 1.2
Whispering Willow East Wind Project [Member] | IPL [Member]
    
Production Tax Credits [Line Items]    
Production Tax Credits $ 4.6 $ 3.9