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Fair Value Measurements (Fair Value Measurements) (Details) - USD ($) $ in Millions | Jun. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Assets: |  |
Derivative assets | $ 32.0 | $ 33.0
Capitalization and liabilities: |  |
Derivative liabilities | 44.0 | 32.0
IPL [Member] |  |
Assets: |  |
Derivative assets | 29.3 | 24.7
Capitalization and liabilities: |  |
Derivative liabilities | 19.3 | 16.2
WPL [Member] |  |
Assets: |  |
Derivative assets | 2.7 | 8.3
Capitalization and liabilities: |  |
Derivative liabilities | 24.7 | 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 | 198.9 | 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 | 198.9 | 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) | 4,210.7 | 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) | 1,937.2 | 2,053.0
Cumulative preferred stock | 0.0 | 0.0
Level 2 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,824.0 | 1,908.9
Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 3.0 | 2.6
Capitalization and liabilities: |  |
Derivative liabilities | 15.6 | 19.5
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 1.9 | 2.4
Capitalization and liabilities: |  |
Derivative liabilities | 10.2 | 13.3
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 1.1 | 0.2
Capitalization and liabilities: |  |
Derivative liabilities | 5.4 | 6.2
Level 3 [Member] |  |
Assets: |  |
Deferred proceeds | 73.4 | 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 | 73.4 | 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 | 32.0 | 36.0
Capitalization and liabilities: |  |
Derivative liabilities | 31.4 | 18.1
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 28.8 | 25.6
Capitalization and liabilities: |  |
Derivative liabilities | 10.5 | 6.2
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 3.2 | 10.4
Capitalization and liabilities: |  |
Derivative liabilities | 20.9 | 11.9
Fair Value [Member] |  |
Assets: |  |
Deferred proceeds | 73.4 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 4,214.4 | 4,418.2
Cumulative preferred stock | 198.9 | 200.2
Fair Value [Member] | IPL [Member] |  |
Assets: |  |
Deferred proceeds | 73.4 | 177.2
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,937.2 | 2,053.0
Cumulative preferred stock | 198.9 | 200.2
Fair Value [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,824.0 | 1,908.9
Fair Value [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 35.0 | 38.6
Capitalization and liabilities: |  |
Derivative liabilities | 47.0 | 37.6
Fair Value [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 30.7 | 28.0
Capitalization and liabilities: |  |
Derivative liabilities | 20.7 | 19.5
Fair Value [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 4.3 | 10.6
Capitalization and liabilities: |  |
Derivative liabilities | $ 26.3 | $ 18.1

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