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Fair Value Measurements (Fair Value Of Financial Instruments) (Details) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Assets: |  |
Derivative assets | $ 20.3 | $ 38.6
Capitalization and liabilities: |  |
Derivative liabilities | 59.1 | 37.6
IPL [Member] |  |
Assets: |  |
Derivative assets | 18.5 | 28.0
Capitalization and liabilities: |  |
Derivative liabilities | 22.0 | 19.5
WPL [Member] |  |
Assets: |  |
Derivative assets | 1.8 | 10.6
Capitalization and liabilities: |  |
Derivative liabilities | 37.1 | 18.1
Carrying Amount [Member] |  |
Assets: |  |
Derivative assets | 20.3 | 38.6
Deferred proceeds (sales of receivables) | 195.5 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,858.8 | 3,789.7
Cumulative preferred stock | 200.0 | 200.0
Derivative liabilities | 59.1 | 37.6
Carrying Amount [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 18.5 | 28.0
Deferred proceeds (sales of receivables) | 195.5 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,868.5 | 1,768.7
Cumulative preferred stock | 200.0 | 200.0
Derivative liabilities | 22.0 | 19.5
Carrying Amount [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 1.8 | 10.6
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,543.6 | 1,573.9
Derivative liabilities | 37.1 | 18.1
Fair Value [Member] |  |
Assets: |  |
Derivative assets | 20.3 | 38.6
Deferred proceeds (sales of receivables) | 195.5 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 4,394.3 | 4,418.2
Cumulative preferred stock | 201.3 | 200.2
Derivative liabilities | 59.1 | 37.6
Fair Value [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 18.5 | 28.0
Deferred proceeds (sales of receivables) | 195.5 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 2,115.3 | 2,053.0
Cumulative preferred stock | 201.3 | 200.2
Derivative liabilities | 22.0 | 19.5
Fair Value [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 1.8 | 10.6
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,825.5 | 1,908.9
Derivative liabilities | $ 37.1 | $ 18.1

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