|
Income Taxes (Production Tax Credits (Net Of State Tax Impacts)) (Details) (USD $)
In Millions, unless otherwise specified |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
|
Sep. 30, 2012
|
Sep. 30, 2011
|
Sep. 30, 2012
|
Sep. 30, 2011
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Alliant Energy [Member]
|
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| Production Tax Credits [Line Items] | ||||
| Production tax credits (net of state tax impacts) | $ 4.5 | $ 3.8 | $ 17.7 | $ 18.1 |
|
WPL [Member]
|
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| Production Tax Credits [Line Items] | ||||
| Production tax credits (net of state tax impacts) | 2.5 | 2.0 | 9.0 | 9.9 |
|
Cedar Ridge [Member] | WPL [Member]
|
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| Production Tax Credits [Line Items] | ||||
| Production tax credits (net of state tax impacts) | 0.7 | 0.6 | 3.0 | 3.2 |
|
Bent Tree - Phase I [Member] | WPL [Member]
|
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| Production Tax Credits [Line Items] | ||||
| Production tax credits (net of state tax impacts) | 1.8 | 1.4 | 6.0 | 6.7 |
|
Whispering Willow - East Wind Project [Member] | IPL [Member]
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| Production Tax Credits [Line Items] | ||||
| Production tax credits (net of state tax impacts) | $ 2.0 | $ 1.8 | $ 8.7 | $ 8.2 |