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Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Public Utility, Property, Plant and Equipment [Line Items]    
Regulatory liabilities utilized $ (74.9) $ 5.0
Increase in regulatory assets 14.4 73.5
Wholesale power supply agreement termination notice minimum term (in years) 5 years  
Wholesale power supply agreement minimum term (in years) 10 years  
Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Regulatory liabilities utilized 67.0  
IPL [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Regulatory liabilities utilized (68.3) (27.5)
Increase in regulatory assets 9.8 57.7
IPL [Member] | Electric tax benefit rider [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Regulatory liabilities utilized 59.0  
IPL [Member] | Gas tax benefit rider [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Regulatory liabilities utilized 8.0  
WPL [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Regulatory liabilities utilized (6.6) 32.5
Increase in regulatory assets 4.6 15.8
Wpl's Edgewater Unit 5 Purchase [Member] | WPL [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Jointly owned utility plant, proportionate ownership share 25.00%  
2013 and 2014 Test Period Retail Electric And Gas Final Order [Member] | Alliant Energy and WPL [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Increase in regulatory assets   5.0
Credit to utility other operation and maintenance   5
IPL FERC Authorized Return On Common Equity [Member] | IPL [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Return on common equity wholesale electric rate 10.97%  
Scenario, Forecast [Member] | IPL [Member]
    
Public Utility, Property, Plant and Equipment [Line Items]    
Expected proceeds from asset sales $ 128