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Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Assets: |  |
Derivative assets | $ 32.3 | $ 23.5
Deferred proceeds | 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 | 4.9 | 17.6
IPL [Member] |  |
Assets: |  |
Derivative assets | 9.7 | 19.5
Deferred proceeds | 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 | 0.9 | 3.6
WPL [Member] |  |
Assets: |  |
Derivative assets | 22.6 | 4.0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,582.5 | 1,532.9
Derivative liabilities | 4.0 | 14.0
Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 41.3 | 26.7
Capitalization and liabilities: |  |
Derivative liabilities | 9.0 | 20.8
Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 15.9 | 21.1
Capitalization and liabilities: |  |
Derivative liabilities | 2.2 | 5.2
Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 25.4 | 5.6
Capitalization and liabilities: |  |
Derivative liabilities | 6.8 | 15.6
Level 1 [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Cumulative preferred stock | 183.8 | 167.0
Level 1 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Cumulative preferred stock | 183.8 | 167.0
Level 1 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 0 | 0
Capitalization and liabilities: |  |
Derivative liabilities | 0 | 0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 0 | 0
Capitalization and liabilities: |  |
Derivative liabilities | 0 | 0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 0 | 0
Capitalization and liabilities: |  |
Derivative liabilities | 0 | 0
Level 2 [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,816.5 | 3,711.8
Cumulative preferred stock | 0 | 0
Level 2 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,779.6 | 1,726.4
Cumulative preferred stock | 0 | 0
Level 2 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,582.5 | 1,532.9
Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 11.2 | 4.7
Capitalization and liabilities: |  |
Derivative liabilities | 2.8 | 3.2
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 6.8 | 3.0
Capitalization and liabilities: |  |
Derivative liabilities | 0.9 | 1.7
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 4.4 | 1.7
Capitalization and liabilities: |  |
Derivative liabilities | 1.9 | 1.5
Level 3 [Member] |  |
Assets: |  |
Deferred proceeds | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3.3 | 0.5
Cumulative preferred stock | 0 | 0
Level 3 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 170.8 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Cumulative preferred stock | 0 | 0
Level 3 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 30.1 | 22.0
Capitalization and liabilities: |  |
Derivative liabilities | 6.2 | 17.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 9.1 | 18.1
Capitalization and liabilities: |  |
Derivative liabilities | 1.3 | 3.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 21.0 | 3.9
Capitalization and liabilities: |  |
Derivative liabilities | $ 4.9 | $ 14.1

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