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Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018
Common Equity [Line Items] |  |  |  |
Proceeds from issuance of common stock, net | $ 54.6 | $ 6.3 |  |
Comprehensive income | 0.7 |  |  |
IPL [Member] |  |  |  |
Common Equity [Line Items] |  |  |  |
Comprehensive income | 0.0 | 0.0 |  |
WPL [Member] |  |  |  |
Common Equity [Line Items] |  |  |  |
Comprehensive income | $ 0.0 | $ 0.0 |  |
Equity Forward Agreements [Member] |  |  |  |
Common Equity [Line Items] |  |  |  |
Common stock issued during the period, equity forward agreements (in shares) | 1,090,300 |  |  |
Forward sales price (in dollars per share) | $ 44.13 |  |  | $ 44.33
Proceeds from issuance of common stock, net | $ 48.0 |  |  |
Shares included in diluted earnings per share (in shares) | 96,261 |  |  |
Scenario, 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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