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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, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2018
Common Equity [Line Items] |  |  |  |  |
Dividends declared per common share (in dollars per share) | $ 0.335 | $ 0.315 | $ 1.005 | $ 0.945 |
Proceeds from issuance of common stock, net |  |  | $ 191.3 | $ 143.2 |
IPL [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Comprehensive income | $ 0.0 | $ 0.0 | 0.0 | 0.0 |
WPL [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Comprehensive income | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 |
At The Market Offering Program [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Common stock issued during the period, At-the-market offering program (in shares) |  |  | 4,171,013 |  |
Proceeds from issuance of common stock, net |  |  | $ 173.0 |  |
Fees and commissions from issuance of common stock |  |  | $ 2.0 |  |
Scenario, Forecast [Member] | At The Market Offering Program [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Maximum aggregate gross sales price of common stock that can be offered and sold |  |  |  |  | $ 175.0

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