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Debt (Credit Facilities) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Commercial paper:    
Amount outstanding $ 217.5 $ 102.8
Weighted average interest rates 0.40% 0.30%
Weighted average remaining maturity 11 days 3 days
Available credit facility capacity 732.5 [1] 897.2 [1]
IPL [Member]
    
Commercial paper:    
Amount outstanding 26.3 7.1
Weighted average interest rates 0.40% 0.40%
Weighted average remaining maturity 2 days 3 days
Available credit facility capacity 223.7 [1] 292.9 [1]
WPL [Member]
    
Commercial paper:    
Amount outstanding 86.6 25.7
Weighted average interest rates 0.30% 0.30%
Weighted average remaining maturity 19 days 3 days
Available credit facility capacity 313.4 [1] 374.3 [1]
Long-term debt [Member]
    
Commercial paper:    
Amount outstanding 50.0 [2] 0 [2]
Long-term debt [Member] | IPL [Member]
    
Commercial paper:    
Amount outstanding $ 50.0 [2] $ 0 [2]
[1] Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt at December 31, 2012. Refer to Note 9(b) for further discussion of $50.0 million of commercial paper outstanding at December 31, 2012 classified as long-term debt on Alliant Energy’s and IPL’s Consolidated Balance Sheets.
[2] As of December 31, 2012, $50.0 million of commercial paper was recorded in “Long-term debt, net” on Alliant Energy’s and IPL’s Consolidated Balance Sheets due to the existence of long-term credit facilities that back-stop this commercial paper balance, along with Alliant Energy’s and IPL’s intent and ability to refinance these balances on a long-term basis. As of December 31, 2012, this commercial paper balance had a remaining maturity of 8 days.