v2.4.0.6

Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | 1 Months Ended | 3 Months Ended | 9 Months Ended |  | 1 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 1 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended
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Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011 | Dec. 31, 2011 2012 Test Year Retail Electric Fuel Related, Final Order [Member] Cross State Air Pollution Rule [Member] | Sep. 30, 2012 WPL [Member] | Sep. 30, 2011 WPL [Member] | Sep. 30, 2012 WPL [Member] | Sep. 30, 2011 WPL [Member] | Sep. 30, 2012 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member] | May 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas, Request [Member] | Jun. 30, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member] | Sep. 30, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member] | Oct. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Oral Decision [Member] | Dec. 31, 2011 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Final Order [Member] | Sep. 30, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Final Order [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2011 IPL [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2011 IPL [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] | Sep. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] Tax Benefit Rider [Member] | Aug. 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Settlement [Member] | Jul. 31, 2010 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] | May 31, 2010 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] Higher Base Rates [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] Renewable Energy Rider [Member] | Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] IPL Electric Transmission Assets Sale [Member] | Sep. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] Sutherland 4 [Member] | Sep. 30, 2012 Alliant Energy and IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] | Sep. 30, 2012 Alliant Energy and IPL [Member] 2011 Test Year Retail Gas IA, Request [Member]
Utility Rate Cases [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in annual rates for its customers, amount |  |  |  |  |  |  |  |  |  |  | $ 13 | $ 25 |  | $ 29 |  |  |  |  |  |  |  |  | $ 36 |  |  |  |  |  |  |  |  |  |
Jointly owned utility plant, proportionate ownership share |  |  |  |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual rates for its customers, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  | 15 |  | 11 |  | 15 | 11 | 8 | 2 | 1 |  |  |
Increase in annual rates for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  |  |  | 22.00% |  |  |  |  |  |  |
Increase in interim rates for its customers, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 9 |  |  |  | 14 |  |  |  |  |  |  |  |
Increase in interim rates for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase for its customers, actual amount | 46.8 | 46.4 | 263.9 | 342.5 |  | 17.2 | 18.9 | 114.7 | 144.4 |  |  |  |  |  |  |  | 29.6 | 27.5 | 149.2 | 198.1 |  |  |  |  |  |  |  |  |  |  |  | 2.0 | 2.0
Recovery of previously incurred costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |
Reduction in annual rates for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate at which fuel costs will be monitored |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred compliance costs tolerance band |  |  |  |  | 0.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Customer refund liability, current |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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