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Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013
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Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | $ 0.6 | $ (13.1) | $ 17.9 | $ 101.2 | $ 23.9 | $ 4.4
Excluding Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative liabilities | (28.3) |  | (7.0) |  |  |
Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | 28.9 |  | 24.9 |  |  |
IPL [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | 18.3 | (5.0) | 19.4 | 64.2 | 7.8 | 14.6
IPL [Member] | Excluding Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative liabilities | (9.0) |  | (3.2) |  |  |
IPL [Member] | Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | 27.3 |  | 22.6 |  |  |
WPL [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | (17.7) | $ (8.1) | (1.5) | $ 37.0 | $ 16.1 | $ (10.2)
WPL [Member] | Excluding Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative liabilities | (19.3) |  | (3.8) |  |  |
WPL [Member] | Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | $ 1.6 |  | $ 2.3 |  |  |

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