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Redeemable Preferred Stock (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 1 Months Ended 9 Months Ended 1 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 1 Months Ended 9 Months Ended 1 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
IPL [Member]
Sep. 30, 2012
IPL [Member]
Sep. 30, 2013
IPL [Member]
Q
board_member
Sep. 30, 2012
IPL [Member]
Dec. 31, 2012
IPL [Member]
Mar. 31, 2013
IPL [Member]
Redeemable Preferred Stock [Member]
5.1% [Member]
Sep. 30, 2013
IPL [Member]
Redeemable Preferred Stock [Member]
5.1% [Member]
Mar. 31, 2013
IPL [Member]
Redeemable Preferred Stock [Member]
8.375% [Member]
Sep. 30, 2013
Alliant Energy and IPL [Member]
Redeemable Preferred Stock [Member]
5.1% [Member]
Sep. 30, 2013
Alliant Energy and IPL [Member]
Redeemable Preferred Stock [Member]
8.375% [Member]
Sep. 30, 2013
WPL [Member]
Sep. 30, 2012
WPL [Member]
Sep. 30, 2013
WPL [Member]
Sep. 30, 2012
WPL [Member]
Dec. 31, 2012
WPL [Member]
Mar. 31, 2013
WPL [Member]
Redeemable Preferred Stock [Member]
4.40% [Member]
Mar. 31, 2013
WPL [Member]
Redeemable Preferred Stock [Member]
6.50% [Member]
Mar. 31, 2013
WPL [Member]
Redeemable Preferred Stock [Member]
4.40% through 6.50% [Member]
Sep. 30, 2013
Alliant Energy and WPL [Member]
Redeemable Preferred Stock [Member]
4.40% through 6.50% [Member]
Mar. 31, 2013
Long-term Debt [Member]
IPL [Member]
Redeemable Preferred Stock [Member]
5.1% [Member]
Temporary Equity [Line Items]                                                
Preferred stock issued during period (in shares)                   8,000,000                            
Preferred stock series             5.10%   8.375% 5.10% 5.10% 8.375%             4.50% 4.40% 6.50%      
Preferred stock issued during period, value                   $ 200                            
Reduction in commercial paper     34.8 17.6     (11.3) 42.9                 56.8 (25.7)           40.0
Preferred stock issuance costs                         5                      
Preferred stock redemption terms                   On or after March 15, 2018                            
Preferred stock redemption price (in dollars per share)                     $ 25                          
Number of board members to be elected (in members)             2                                  
Number of quarterly dividend requirements (in quarters)             6                                  
Preferred stock redeemed during period (in shares)                       6,000,000                   1,049,225    
Stock redeemed during period, value                       150                   61    
Preferred stock issuance charge $ 2.6 $ 4.0 $ 15.3 $ 11.9 $ 2.6 $ 3.2 $ 13.7 $ 9.4           $ 5.0 $ 0 $ 0.8 $ 1.6 $ 2.5         $ 1.0