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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 3 Months Ended
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Sep. 30, 2013 wind_projects | Sep. 30, 2012 | Sep. 30, 2013 Deferred income tax assets [Member] | Sep. 30, 2013 IPL [Member] | Sep. 30, 2012 IPL [Member] | Sep. 30, 2013 IPL [Member] Whispering Willow - East Wind Project [Member] MW | Sep. 30, 2013 WPL [Member] | Sep. 30, 2013 WPL [Member] Cedar Ridge Wind Project [Member] MW | Sep. 30, 2013 WPL [Member] Bent Tree - Phase I Wind Project [Member] MW | Sep. 30, 2013 WPL [Member] Deferred income tax assets [Member] | Mar. 31, 2012 State Apportionment [Member] | Mar. 31, 2012 State Apportionment [Member] IPL [Member] | Mar. 31, 2012 State Apportionment [Member] WPL [Member]
Income Tax [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Wind projects generating production tax credits (in wind projects) | 3 |  |  |  |  |  |  |  |  |  |  |  |
Electric capacity of wind projects generating production tax credits (in megawatts) |  |  |  |  |  | 200 |  | 68 | 200 |  |  |  |
Deferred income tax expense |  |  |  |  |  |  |  |  |  |  | $ 15 | $ 8 | $ 7
Increase (decrease) current deferred tax assets | (75.4) | (90.5) | (39.0) | (71.0) | (88.9) |  |  |  |  | (40.7) |  |  |
Increase (decrease) non-current deferred tax liabilities | $ 129.7 |  |  | $ 73.7 |  |  | $ 41.3 |  |  |  |  |  |

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