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Derivative Instruments (Gains And Losses From Derivative Instruments Not Designated As Hedging Instruments) (Details) (USD $)
In Millions, unless otherwise specified |
3 Months Ended | 9 Months Ended | ||
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Sep. 30, 2012
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Sep. 30, 2011
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Sep. 30, 2012
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Sep. 30, 2011
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Regulatory Assets [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||||
| Gains (losses) on derivative instruments | $ (6.3) | $ (23.8) | $ (38.3) | $ (33.1) |
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Regulatory Assets [Member] | IPL [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||||
| Gains (losses) on derivative instruments | (0.1) | (13.3) | (17.3) | (18.7) |
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Regulatory Assets [Member] | WPL [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||||
| Gains (losses) on derivative instruments | (6.2) | (10.5) | (21.0) | (14.4) |
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Regulatory Liabilities [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||||
| Gains (losses) on derivative instruments | 15.8 | 3.7 | 21.3 | 8.6 |
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Regulatory Liabilities [Member] | IPL [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||||
| Gains (losses) on derivative instruments | 6.9 | 2.2 | 11.3 | 5.4 |
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Regulatory Liabilities [Member] | WPL [Member]
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| Derivative Instruments, Gain (Loss) [Line Items] | ||||
| Gains (losses) on derivative instruments | $ 8.9 | $ 1.5 | $ 10.0 | $ 3.2 |