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Fair Value Measurements (Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified | Jun. 30, 2014 | Dec. 31, 2013
--- | --- | ---
Assets: |  |
Derivative assets | $ 103.9 | $ 23.5
Deferred proceeds | 193.7 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 3,907.4 | 3,712.3
Cumulative preferred stock | 199.2 | 167.0
Derivative liabilities | 6.6 | 17.6
IPL [Member] |  |
Assets: |  |
Derivative assets | 68.8 | 19.5
Deferred proceeds | 193.7 | 203.5
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,821.3 | 1,726.4
Cumulative preferred stock | 199.2 | 167.0
Derivative liabilities | 2.4 | 3.6
WPL [Member] |  |
Assets: |  |
Derivative assets | 35.1 | 4.0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,627.1 | 1,532.9
Derivative liabilities | 4.2 | 14.0
Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 119.0 | 26.7
Capitalization and liabilities: |  |
Derivative liabilities | 13.3 | 20.8
Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 73.0 | 21.1
Capitalization and liabilities: |  |
Derivative liabilities | 6.3 | 5.2
Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 46.0 | 5.6
Capitalization and liabilities: |  |
Derivative liabilities | 7.0 | 15.6
Level 1 [Member] |  |
Assets: |  |
Deferred proceeds | 0 | 0
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 0 | 0
Cumulative preferred stock | 199.2 | 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 | 199.2 | 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) | 3,904.1 | 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) | 1,821.3 | 1,726.4
Cumulative preferred stock | 0 | 0
Level 2 [Member] | WPL [Member] |  |
Capitalization and liabilities: |  |
Long-term debt (including current maturities) | 1,627.1 | 1,532.9
Level 2 [Member] | Commodity Contracts [Member] |  |
Assets: |  |
Derivative assets | 10.6 | 4.7
Capitalization and liabilities: |  |
Derivative liabilities | 6.1 | 3.2
Level 2 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 6.7 | 3.0
Capitalization and liabilities: |  |
Derivative liabilities | 4.2 | 1.7
Level 2 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 3.9 | 1.7
Capitalization and liabilities: |  |
Derivative liabilities | 1.9 | 1.5
Level 3 [Member] |  |
Assets: |  |
Deferred proceeds | 193.7 | 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 | 193.7 | 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 | 108.4 | 22.0
Capitalization and liabilities: |  |
Derivative liabilities | 7.2 | 17.6
Level 3 [Member] | Commodity Contracts [Member] | IPL [Member] |  |
Assets: |  |
Derivative assets | 66.3 | 18.1
Capitalization and liabilities: |  |
Derivative liabilities | 2.1 | 3.5
Level 3 [Member] | Commodity Contracts [Member] | WPL [Member] |  |
Assets: |  |
Derivative assets | 42.1 | 3.9
Capitalization and liabilities: |  |
Derivative liabilities | $ 5.1 | $ 14.1

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