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Fair Value Measurements (Fair Value Of Net Derivative Assets (Liabilities)) (Details) - Commodity Contracts [Member] - USD ($) $ in Millions | Sep. 30, 2015 | Jun. 30, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Dec. 31, 2013
--- | --- | --- | --- | --- | --- | ---
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | $ (24.6) | $ 0.6 | $ 17.9 | $ 68.3 | $ 101.2 | $ 4.4
Excluding Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative liabilities | (39.4) |  | (7.0) |  |  |
Financial Transmission Rights [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | 14.8 |  | 24.9 |  |  |
IPL [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | 3.8 | 18.3 | 19.4 | 36.5 | 64.2 | 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 | (10.9) |  | (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 | 14.7 |  | 22.6 |  |  |
WPL [Member] |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |  |  |
Fair value, net derivative assets | (28.4) | $ (17.7) | (1.5) | $ 31.8 | $ 37.0 | $ (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 | (28.5) |  | (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 | $ 0.1 |  | $ 2.3 |  |  |

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