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Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Assets: |  |
Derivative assets | $ 18.0 | $ 33.0
Capitalization and liabilities: |  |
Derivative liabilities | 56.8 | 32.0
IPL [Member] |  |
Assets: |  |
Derivative assets | 17.3 | 24.7
Capitalization and liabilities: |  |
Derivative liabilities | 20.8 | 16.2
WPL [Member] |  |
Assets: |  |
Derivative assets | 0.7 | 8.3
Capitalization and liabilities: |  |
Derivative liabilities | 36.0 | 15.8
Level 1 [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
Cumulative preferred stock | 201.3 | 200.2
Level 1 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
Cumulative preferred stock | 201.3 | 200.2
Level 1 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
Level 1 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 0.0 | 0.0
Capitalization and liabilities: |  |
Derivative liabilities | 0.0 | 0.0
Level 1 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 0.0 | 0.0
Capitalization and liabilities: |  |
Derivative liabilities | 0.0 | 0.0
Level 1 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 0.0 | 0.0
Capitalization and liabilities: |  |
Derivative liabilities | 0.0 | 0.0
Level 2 [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 4,390.6 | 4,414.9
Cumulative preferred stock | 0.0 | 0.0
Level 2 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 0.0 | 0.0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 2,115.3 | 2,053.0
Cumulative preferred stock | 0.0 | 0.0
Level 2 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,825.5 | 1,908.9
Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 2.6 | 2.6
Capitalization and liabilities: |  |
Derivative liabilities | 16.8 | 19.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 2.1 | 2.4
Capitalization and liabilities: |  |
Derivative liabilities | 9.4 | 13.3
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 0.5 | 0.2
Capitalization and liabilities: |  |
Derivative liabilities | 7.4 | 6.2
Level 3 [Member] |  |
Assets: |  |
Deferred proceeds | 195.5 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3.7 | 3.3
Cumulative preferred stock | 0.0 | 0.0
Level 3 [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 195.5 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
Cumulative preferred stock | 0.0 | 0.0
Level 3 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0.0 | 0.0
Level 3 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 17.7 | 36.0
Capitalization and liabilities: |  |
Derivative liabilities | 42.3 | 18.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 16.4 | 25.6
Capitalization and liabilities: |  |
Derivative liabilities | 12.6 | 6.2
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 1.3 | 10.4
Capitalization and liabilities: |  |
Derivative liabilities | 29.7 | 11.9
Fair Value [Member] |  |
Assets: |  |
Deferred proceeds | 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
Fair Value [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 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
Fair Value [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,825.5 | 1,908.9
Fair Value [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 20.3 | 38.6
Capitalization and liabilities: |  |
Derivative liabilities | 59.1 | 37.6
Fair Value [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 18.5 | 28.0
Capitalization and liabilities: |  |
Derivative liabilities | 22.0 | 19.5
Fair Value [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 1.8 | 10.6
Capitalization and liabilities: |  |
Derivative liabilities | $ 37.1 | $ 18.1

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