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Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Assets: |  |
Derivative assets | $ 23.5 | $ 19.3
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,348.4 | 3,149.9
Long-term debt (including current maturities) | 3,712.3 | 3,860.5
Cumulative preferred stock |  | 212.6
Cumulative preferred stock | 200.0 | 145.1
Cumulative preferred stock | 167.0 |
Cumulative preferred stock | 0 | 60.0
Derivative liabilities | 17.6 | 33.5
IPL [Member] |  |
Assets: |  |
Derivative assets | 19.5 | 14.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,563.4 | 1,363.4
Long-term debt (including current maturities) | 1,726.4 | 1,679.9
Cumulative preferred stock | 200.0 | 145.1
Cumulative preferred stock | 167.0 | 151.8
Derivative liabilities | 3.6 | 13.1
WPL [Member] |  |
Assets: |  |
Derivative assets | 4.0 | 4.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,339.1 | 1,339.1
Long-term debt (including current maturities) | 1,532.9 | 1,713.3
Cumulative preferred stock | 0 | 60.8
Cumulative preferred stock | 0 | 60.0
Derivative liabilities | 14.0 | 20.4
Carrying Amount [Member] |  |
Assets: |  |
Derivative assets | 26.7 | 26.2
Deferred proceeds (sales of receivables) | 203.5 | 66.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,336.3 | 3,138.1
Cumulative preferred stock |  | 205.1
Cumulative preferred stock | 200.0 |
Derivative liabilities | 20.8 | 40.4
Carrying Amount [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 21.1 | 17.5
Deferred proceeds (sales of receivables) | 203.5 | 66.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,558.4 | 1,359.5
Cumulative preferred stock | 200.0 | 145.1
Derivative liabilities | 5.2 | 16.1
Carrying Amount [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 5.6 | 8.7
Deferred proceeds (sales of receivables) | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,332.1 | 1,331.5
Cumulative preferred stock | 0 | 60.0
Derivative liabilities | 15.6 | 24.3
Fair Value [Member] |  |
Assets: |  |
Derivative assets | 26.7 | 26.2
Deferred proceeds (sales of receivables) | 203.5 | 66.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,712.3 | 3,860.5
Cumulative preferred stock |  | 212.6
Cumulative preferred stock | 167.0 |
Derivative liabilities | 20.8 | 40.4
Fair Value [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 21.1 | 17.5
Deferred proceeds (sales of receivables) | 203.5 | 66.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,726.4 | 1,679.9
Cumulative preferred stock | 167.0 | 151.8
Derivative liabilities | 5.2 | 16.1
Fair Value [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 5.6 | 8.7
Deferred proceeds (sales of receivables) | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,532.9 | 1,713.3
Cumulative preferred stock | 0 | 60.8
Derivative liabilities | $ 15.6 | $ 24.3

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