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Debt (Credit Facilities) (Details) (USD $)
In Millions, unless otherwise specified
 9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Commercial paper:    
Amount outstanding $ 70.4 $ 102.8
Weighted average remaining maturity (in days) 0 years 0 months 1 day  
Weighted average interest rates 0.40%  
Available credit facility capacity 879.6 [1]  
Parent Company [Member]
    
Commercial paper:    
Amount outstanding 70.4  
Weighted average remaining maturity (in days) 0 years 0 months 1 day  
Weighted average interest rates 0.40%  
Available credit facility capacity 229.6 [1]  
IPL [Member]
    
Commercial paper:    
Amount outstanding 0 7.1
Available credit facility capacity 250.0 [1]  
WPL [Member]
    
Commercial paper:    
Amount outstanding 0 25.7
Available credit facility capacity $ 400.0 [1]  
[1] Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt at September 30, 2012. Refer to Note 7(b) for further discussion of $50 million of commercial paper outstanding at September 30, 2012 classified as long-term debt.