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Income Taxes (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2018
Income Taxes [Line Items] |  |
Income tax benefit, Federal Tax Reform | $ 5.6 |
Increase in deferred tax liabilities |  | $ 96.8
IPL [Member] |  |
Income Taxes [Line Items] |  |
Income tax benefit, Federal Tax Reform | 1.1 |
Increase in deferred tax liabilities |  | 22.3
WPL [Member] |  |
Income Taxes [Line Items] |  |
Income tax benefit, Federal Tax Reform | $ 5.5 |
Increase in deferred tax liabilities |  | $ 57.2

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