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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
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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% |  |  |  |  |  |  |  |  |  |  |

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