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Utility Rate Cases (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 1 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
WPL [Member]
Jun. 30, 2012
WPL [Member]
Jun. 30, 2013
WPL [Member]
Jun. 30, 2012
WPL [Member]
Jul. 31, 2012
WPL [Member]
2013/2014 Test Period Retail Electric And Gas, Request [Member]
Jul. 31, 2013
WPL [Member]
2014 Test Year Retail Electric Fuel Related, Request [Member]
Dec. 31, 2012
WPL [Member]
2013 Test Year Retail Electric Fuel Related, Oral Decision [Member]
Jun. 30, 2013
WPL [Member]
2013 Test Year Retail Electric Fuel Related, Request [Member]
Jun. 30, 2013
IPL [Member]
Jun. 30, 2012
IPL [Member]
Jun. 30, 2013
IPL [Member]
Jun. 30, 2012
IPL [Member]
Jun. 30, 2013
IPL [Member]
Gas tax benefit rider [Member]
Jun. 30, 2012
IPL [Member]
2011 Test Year Retail Gas IA, Request [Member]
May 31, 2012
IPL [Member]
2011 Test Year Retail Gas IA, Request [Member]
Gas tax benefit rider [Member]
Nov. 30, 2012
IPL [Member]
2011 Test Year Retail Gas IA, Settlement [Member]
Jun. 30, 2013
Alliant Energy and IPL [Member]
Revenue Requirement Adjustment [Member]
Jun. 30, 2013
Alliant Energy and IPL [Member]
Revenue Requirement Adjustment [Member]
Dec. 31, 2013
Scenario, Forecast [Member]
IPL [Member]
Revenue Requirement Adjustment [Member]
Utility Rate Cases [Line Items]                                              
Reduction in annual rates for its customers, amount                 $ 13   $ 29               $ 36        
Proposed gas tax benefit rider, term                                     3 years        
Increase in interim rates for its customers, amount                                   9          
Increase in interim rates for its customers, percentage                                   3.00%          
Increase in annual rates for its customers, amount                   31                   11      
Increase in annual rates for its customers, percentage                   3.00%                   4.00%      
Revenue requirement adjustment     15.1 43.5     (12.0) (18.0)             27.1 61.5 (5.0)           (24.0)
Electric revenue $ 612.1 $ 612.6 $ 1,245.3 $ 1,185.0 $ 282.5 $ 291.6 $ 565.5 $ 570.9         $ 329.6 $ 321.0 $ 679.8 $ 614.1         $ 5.0 $ 11.0  
Reduction in annual rates for its customers, percentage                     3.00%                        
Rate at which fuel costs will be monitored                       2.00%