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Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 1 Months Ended |  | 3 Months Ended | 9 Months Ended | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
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Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012 | Dec. 31, 2012 2012 Test Year Retail Electric Fuel Related, Request [Member] | Sep. 30, 2013 WPL [Member] | Sep. 30, 2012 WPL [Member] | Sep. 30, 2013 WPL [Member] | Sep. 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, Final Order [Member] | Sep. 30, 2013 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member] | Sep. 30, 2013 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Request [Member] | Dec. 31, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Request [Member] | Sep. 30, 2013 IPL [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2013 IPL [Member] | Sep. 30, 2012 IPL [Member] | Sep. 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] | Sep. 30, 2013 Alliant Energy and IPL [Member] Revenue Requirement Adjustment [Member] | Sep. 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 |  |  | 74.9 | (5.0) |  |  |  | 6.6 | (32.5) |  |  |  |  |  |  |  |  | 68.3 | 27.5 | (8.0) |  |  |  |  |  | 24.0
Electric revenue | 798.1 | 815.3 | 2,043.4 | 2,000.3 |  | 340.5 | 358.7 | 906.0 | 929.6 |  |  |  |  |  |  | 457.6 | 456.6 | 1,137.4 | 1,070.7 |  |  |  |  | 7.0 | 18.0 |
Reduction in annual rates for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate at which fuel costs will be monitored |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Over-collection of fuel-related costs |  |  |  |  | 17 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collected outside the approved range for 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11 |  |  |  |  |  |  |  |  |  |  |
Refunds to customers, including interest |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 12 |  |  |  |  |  |  |  |  |  |  |  |

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