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Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended | 12 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012 | Mar. 31, 2013 Revenue Requirement Adjustment [Member] | Mar. 31, 2013 WPL [Member] | Mar. 31, 2012 WPL [Member] | Jul. 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas, Request [Member] | Dec. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Oral Decision [Member] | Mar. 31, 2013 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member] | Mar. 31, 2013 IPL [Member] | Mar. 31, 2012 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] Tax Benefit Rider [Member] | Nov. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Settlement [Member] | Dec. 31, 2013 Scenario, Forecast [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 |  |  |  |  |  |  |  |  |  |  |  |  | 11 |
Increase in annual rates for its customers, percentage |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |
Revenue requirement adjustment | 20.4 | (26.5) |  |  |  |  |  |  | 14.7 | (33.6) |  |  |  | (24.0)
Electric revenue | $ 633.2 | $ 572.4 | $ 5.5 | $ 283.0 | $ 279.3 |  |  |  | $ 350.2 | $ 293.1 |  |  |  |
Reduction in annual rates for its customers, percentage |  |  |  |  |  |  | 3.00% |  |  |  |  |  |  |
Rate at which fuel costs will be monitored |  |  |  |  |  |  |  | 2.00% |  |  |  |  |  |

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