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Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Schedule of Common Equity [Line Items] |  |  |  |
Shares available for issuance under the Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan (in shares) |  | 12,100,000 |  |
Proceeds from issuance of common stock, net |  | $ 390.3 | $ 196.6 | $ 149.6
Other comprehensive income (loss) |  | (0.4) | 2.2 | (0.1)
IPL [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Other comprehensive income (loss) |  | 0.0 | 0.0 | 0.0
WPL [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Other comprehensive income (loss) |  | $ 0.0 | $ 0.0 | $ 0.0
Equity Forward Agreements [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Common stock issued during the period (in shares) |  | 8,358,973 | 0 | 0
Proceeds from issuance of common stock, net |  | $ 366.0 |  |
Initial forward sale price (in dollars per share) |  | $ 43.75 |  |
Shares included in diluted earnings per share (in shares) |  | 67,689 |  |
At-the-market Offering Program [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Common stock issued during the period (in shares) |  | 0 | 4,171,013 | 3,074,931
Proceeds from issuance of common stock, net |  |  | $ 173.0 | $ 124.0
Fees and commissions from issuance of common stock |  |  | 2.0 | 1.0
Maximum aggregate gross sales price of common stock that can be offered and sold |  |  | $ 175.0 | $ 125.0
Forecast [Member] | Equity Forward Agreements [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Common stock issued during the period (in shares) | 4,275,127 |  |  |
Initial forward sale price (in dollars per share) | $ 52.235 |  |  |

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