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Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2019 | Dec. 31, 2018
Common Equity [Line Items] |  |  |  |  |  |
Proceeds from issuance of common stock, net | $ 124.8 | $ 91.2 | $ 185.4 | $ 191.3 |  |
Comprehensive income (loss) |  | (0.3) | (0.7) | 0.3 |  |
IPL [Member] |  |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |  |
Comprehensive income (loss) | 0.0 | 0.0 | 0.0 | 0.0 |  |
WPL [Member] |  |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |  |
Comprehensive income (loss) | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 |  |
Equity Forward Agreements [Member] |  |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |  |
Common stock issued during the period, equity forward agreements (in shares) |  |  | 3,790,300 |  |  |
Forward sales price (in dollars per share) | $ 43.99 |  | $ 43.99 |  |  | $ 44.33
Proceeds from issuance of common stock, net |  |  | $ 167.0 |  |  |
Shares included in diluted earnings per share (in shares) |  |  | 671,237 |  |  |
Forecast [Member] | Equity Forward Agreements [Member] |  |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |  |
Common stock issued during the period, equity forward agreements (in shares) |  |  |  |  | 8,358,973 |

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