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Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012
--- | --- | ---
Assets: |  |
Derivative assets | $ 32.3 | $ 19.3
Deferred proceeds | 86.9 | 66.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,553.6 | 3,860.5
Cumulative preferred stock |  | 212.6
Cumulative preferred stock | 169.5 |
Derivative liabilities | 23.5 | 33.0
IPL [Member] |  |
Assets: |  |
Derivative assets | 29.2 | 14.5
Deferred proceeds | 86.9 | 66.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,553.8 | 1,679.9
Cumulative preferred stock | 169.5 | 151.8
Derivative liabilities | 5.3 | 12.6
WPL [Member] |  |
Assets: |  |
Derivative assets | 3.1 | 4.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,541.9 | 1,713.3
Cumulative preferred stock | 0 | 60.8
Derivative liabilities | 18.2 | 20.4
Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 36.8 | 26.2
Capitalization and liabilities: |  |
Derivative liabilities | 27.1 | 40.4
Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 31.5 | 17.5
Capitalization and liabilities: |  |
Derivative liabilities | 6.7 | 16.1
Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 5.3 | 8.7
Capitalization and liabilities: |  |
Derivative liabilities | 20.4 | 24.3
Level 1 [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Cumulative preferred stock |  | 162.3
Cumulative preferred stock | 169.5 |
Level 1 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Cumulative preferred stock | 169.5 | 151.8
Level 1 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Cumulative preferred stock | 0 | 10.5
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) | 3,553.2 | 3,860.0
Cumulative preferred stock |  | 50.3
Cumulative preferred stock | 0 |
Level 2 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,553.8 | 1,679.9
Cumulative preferred stock | 0 | 0
Level 2 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,541.9 | 1,713.3
Cumulative preferred stock | 0 | 50.3
Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 3.2 | 4.8
Capitalization and liabilities: |  |
Derivative liabilities | 10.7 | 30.9
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 1.9 | 3.1
Capitalization and liabilities: |  |
Derivative liabilities | 5.7 | 14.2
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 1.3 | 1.7
Capitalization and liabilities: |  |
Derivative liabilities | 5.0 | 16.7
Level 3 [Member] |  |
Assets: |  |
Deferred proceeds | 86.9 | 66.8
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0.4 | 0.5
Cumulative preferred stock |  | 0
Cumulative preferred stock | 0 |
Level 3 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 86.9 | 66.8
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
Cumulative preferred stock | 0 | 0
Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 33.6 | 21.4
Capitalization and liabilities: |  |
Derivative liabilities | 16.4 | 9.5
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 29.6 | 14.4
Capitalization and liabilities: |  |
Derivative liabilities | 1.0 | 1.9
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 4.0 | 7.0
Capitalization and liabilities: |  |
Derivative liabilities | $ 15.4 | $ 7.6

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