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Redeemable Preferred Stock (Details) (USD $)
In Millions, except Share data, unless otherwise specified |
12 Months Ended | 1 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 1 Months Ended | 3 Months Ended | |||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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Dec. 31, 2012
Redeemable Preferred Stock [Member]
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Dec. 31, 2011
Redeemable Preferred Stock [Member]
|
Dec. 31, 2012
IPL [Member]
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Dec. 31, 2011
IPL [Member]
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Dec. 31, 2010
IPL [Member]
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Dec. 31, 2012
IPL [Member]
Redeemable Preferred Stock [Member]
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Dec. 31, 2011
IPL [Member]
Redeemable Preferred Stock [Member]
|
Dec. 31, 2012
IPL [Member]
Redeemable Preferred Stock [Member]
8.375% [Member]
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Dec. 31, 2011
IPL [Member]
Redeemable Preferred Stock [Member]
8.375% [Member]
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Mar. 31, 2013
IPL [Member]
Redeemable Preferred Stock [Member]
8.375% [Member]
Subsequent Event [Member]
Scenario, Forecast [Member]
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Dec. 31, 2012
WPL [Member]
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Dec. 31, 2011
WPL [Member]
|
Dec. 31, 2010
WPL [Member]
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Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
|
Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
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Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
4.50% [Member]
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Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
4.50% [Member]
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Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
4.80% [Member]
|
Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
4.80% [Member]
|
Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
4.96% [Member]
|
Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
4.96% [Member]
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Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
4.40% [Member]
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Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
4.40% [Member]
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Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
4.76% [Member]
|
Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
4.76% [Member]
|
Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
6.20% [Member]
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Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
6.20% [Member]
|
Dec. 31, 2012
WPL [Member]
Redeemable Preferred Stock [Member]
6.50% [Member]
|
Dec. 31, 2011
WPL [Member]
Redeemable Preferred Stock [Member]
6.50% [Member]
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Mar. 31, 2013
WPL [Member]
Redeemable Preferred Stock [Member]
4.40% through 6.50% [Member]
Subsequent Event [Member]
Scenario, Forecast [Member]
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Mar. 31, 2013
Alliant Energy And IPL [Member]
Redeemable Preferred Stock [Member]
8.375% [Member]
Subsequent Event [Member]
Scenario, Forecast [Member]
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Mar. 31, 2013
Alliant Energy and WPL [Member]
Redeemable Preferred Stock [Member]
4.40% through 6.50% [Member]
Subsequent Event [Member]
Scenario, Forecast [Member]
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| Temporary Equity [Line Items] | |||||||||||||||||||||||||||||||||||||||||||||||||||
| Liquidation Preference/Stated Value (in dollars per share) | $ 25 | ||||||||||||||||||||||||||||||||||||||||||||||||||
| Liquidation Preference/Stated Value (in dollars per share) | $ 100 | [1] | $ 100 | [1] | $ 100 | [1] | $ 100 | [1] | $ 100 | [1] | $ 100 | [1] | $ 25 | [1] | |||||||||||||||||||||||||||||||||||||
| Authorized Shares | 16,000,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||
| Authorized Shares | 3,750,000 | 3,750,000 | 3,750,000 | 3,750,000 | 3,750,000 | 3,750,000 | 3,750,000 | ||||||||||||||||||||||||||||||||||||||||||||
| Shares Outstanding | 6,000,000 | ||||||||||||||||||||||||||||||||||||||||||||||||||
| Shares Outstanding | 99,970 | [1] | 74,912 | [1] | 64,979 | [1] | 29,957 | [1] | 29,947 | [1] | 150,000 | [1] | 599,460 | [1] | |||||||||||||||||||||||||||||||||||||
| Series | 8.375% | 8.375% | 4.50% | 4.50% | [1] | 4.80% | [1] | 4.96% | [1] | 4.40% | [1] | 4.76% | [1] | 6.20% | [1] | 6.50% | [1] | ||||||||||||||||||||||||||||||||||
| Redemption (none are mandatorily redeemable) | On or after March 15, 2013 | Any time | [1] | Any time | [1] | Any time | [1] | Any time | [1] | Any time | [1] | Any time | [1] | Any time | [1] | ||||||||||||||||||||||||||||||||||||
| Cumulative preferred stock value, at par | $ 145.1 | $ 145.1 | $ 145.1 | $ 145.1 | $ 145.1 | $ 145.1 | $ 150.0 | $ 150.0 | |||||||||||||||||||||||||||||||||||||||||||
| Less: discount | (4.9) | (4.9) | |||||||||||||||||||||||||||||||||||||||||||||||||
| Cumulative preferred stock | 60.0 | 60.0 | 60.0 | 60.0 | 60.0 | 60.0 | 10.0 | [1] | 10.0 | [1] | 7.5 | [1] | 7.5 | [1] | 6.5 | [1] | 6.5 | [1] | 3.0 | [1] | 3.0 | [1] | 3.0 | [1] | 3.0 | [1] | 15.0 | [1] | 15.0 | [1] | 15.0 | [1] | 15.0 | [1] | |||||||||||||||||
| Preferred stock and temporary equity net carrying amount | 205.1 | 205.1 | |||||||||||||||||||||||||||||||||||||||||||||||||
| Preferred stock called during period (in shares) | 6,000,000 | 1,049,225 | |||||||||||||||||||||||||||||||||||||||||||||||||
| Stock redeemed during period, value | 150 | 61 | |||||||||||||||||||||||||||||||||||||||||||||||||
| Preferred dividends | $ 15.9 | $ 18.3 | $ 18.7 | $ 12.6 | $ 15.0 | $ 15.4 | $ 3.3 | $ 3.3 | $ 3.3 | $ 5.0 | $ 1.0 | ||||||||||||||||||||||||||||||||||||||||
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