v2.4.0.6

Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  | 1 Months Ended |  | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended |  | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] | Dec. 31, 2012 WPL [Member] | Dec. 31, 2011 WPL [Member] | Dec. 31, 2010 WPL [Member] | Dec. 31, 2012 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member] | May 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas Rate Request [Member] | Dec. 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas Rate Request [Member] WPL's Edgewater Unit 5 Purchase [Member] | Jan. 31, 2011 WPL [Member] 2011 Test Year Retail Electric Request Final Order [Member] | Dec. 31, 2010 WPL [Member] 2011 Test Year Retail Electric Request Final Order [Member] | Dec. 31, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member] | Jun. 30, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member] | Dec. 31, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member] | Dec. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related Request [Member] | Dec. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related Request [Member] | Dec. 31, 2011 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] | Dec. 31, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] | Dec. 31, 2012 IPL [Member] | Dec. 31, 2011 IPL [Member] | Dec. 31, 2010 IPL [Member] | Nov. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [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] | Dec. 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] | Dec. 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] Tax Benefit Rider [Member] | Feb. 28, 2011 IPL [Member] 2009 Test Year Retail Electric IA, Request [Member] | Mar. 31, 2010 IPL [Member] 2009 Test Year Retail Electric IA, Request [Member] | Dec. 31, 2011 IPL [Member] 2009 Test Year Retail Electric IA, Request [Member] | Jul. 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]
Utility Rate Cases [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in annual rates for its customers, amount |  |  |  |  |  |  |  |  | $ 13 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 36 |  |  |  |  |  |  |  |
Jointly owned utility plant, proportionate ownership share |  |  |  |  |  |  |  | 25.00% |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual rates for its customers, amount |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |  | 4 |  |  |  |  | 11 |  |  |  |  | 114 |  |  |  | 11 | 8 | 2 | 1
Increase in annual rates for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |  |  |  |  |  |  | 4.00% |  |  |  |  | 10.00% |  |  |  |  |  |  |
Increase in interim rates for its customers, amount |  |  |  |  |  |  |  |  |  |  |  |  | 9 | 9 | 5 |  |  |  |  |  |  |  |  | 9 |  |  |  |  | 119 |  | 14 |  |  |  |
Decrease in annual electric retail rates |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  | 29 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in interim rates for its customers, amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase, adjustment period subsequent to test year |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 months |  |  |  | 12 months |  |  |  |  |  |
Proposed gas tax benefit rider, term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |
Increase in interim rates for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |
Gas revenue | 396.3 | 476.7 | 480.6 |  | 169.6 | 200.4 | 206.3 |  |  |  |  |  |  |  |  |  |  |  |  | 226.7 | 276.3 | 274.3 |  |  |  | 5.0 |  |  |  |  |  |  |  |  |
Revenue refunded to customers, including interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 |  |  |  |  |
Decrease in annual rate for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate at which fuel costs will be monitored |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Refunds anticipated to be used to offset fuel related cost changes in 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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