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Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Assets: |  |
Derivative assets | $ 41.3 | $ 26.7
Deferred proceeds (sales of receivables) | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,819.8 | 3,712.3
Cumulative preferred stock | 200.0 | 200.0
Cumulative preferred stock | 183.8 | 167.0
Derivative liabilities | 9.0 | 20.8
IPL [Member] |  |
Assets: |  |
Derivative assets | 15.9 | 21.1
Deferred proceeds (sales of receivables) | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,779.6 | 1,726.4
Cumulative preferred stock | 200.0 | 200.0
Cumulative preferred stock | 183.8 | 167.0
Derivative liabilities | 2.2 | 5.2
WPL [Member] |  |
Assets: |  |
Derivative assets | 25.4 | 5.6
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,582.5 | 1,532.9
Derivative liabilities | 6.8 | 15.6
Carrying Amount [Member] |  |
Assets: |  |
Derivative assets | 41.3 | 26.7
Deferred proceeds (sales of receivables) | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,339.4 | 3,336.3
Cumulative preferred stock | 200.0 | 200.0
Derivative liabilities | 9.0 | 20.8
Carrying Amount [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 15.9 | 21.1
Deferred proceeds (sales of receivables) | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,558.6 | 1,558.4
Cumulative preferred stock | 200.0 | 200.0
Derivative liabilities | 2.2 | 5.2
Carrying Amount [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 25.4 | 5.6
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,332.2 | 1,332.1
Derivative liabilities | 6.8 | 15.6
Fair Value [Member] |  |
Assets: |  |
Derivative assets | 41.3 | 26.7
Deferred proceeds (sales of receivables) | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,819.8 | 3,712.3
Cumulative preferred stock | 183.8 | 167.0
Derivative liabilities | 9.0 | 20.8
Fair Value [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 15.9 | 21.1
Deferred proceeds (sales of receivables) | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,779.6 | 1,726.4
Cumulative preferred stock | 183.8 | 167.0
Derivative liabilities | 2.2 | 5.2
Fair Value [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 25.4 | 5.6
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,582.5 | 1,532.9
Derivative liabilities | $ 6.8 | $ 15.6

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