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Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
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Derivative [Line Items] |  |
Derivative assets, Gross (as reported) | $ 23.6 | $ 26.2
Derivative assets, Net | 16.3 | 19.3
Derivative liabilities, Gross (as reported) | 17.1 | 40.4
Derivative liabilities, Net | 9.8 | 33.0
IPL [Member] |  |
Derivative [Line Items] |  |
Derivative assets, Gross (as reported) | 12.5 | 17.5
Derivative assets, Net | 9.0 | 14.5
Derivative liabilities, Gross (as reported) | 6.1 | 16.1
Derivative liabilities, Net | 2.6 | 12.6
WPL [Member] |  |
Derivative [Line Items] |  |
Derivative assets, Gross (as reported) | 11.1 | 8.7
Derivative assets, Net | 7.3 | 4.8
Derivative liabilities, Gross (as reported) | 11.0 | 24.3
Derivative liabilities, Net | $ 7.2 | $ 20.4

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