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Common Equity (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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) |  | 12,400,000.0 |  |
Proceeds from issuance of common stock, net |  | $ 149.6 | $ 26.6 | $ 151.2
Ownership percentage |  | 15.00% |  |
Other comprehensive income (loss) |  | $ (0.1) | 0.0 | 0.2
IPL [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Retained earnings free of dividend restrictions |  | $ 679.0 |  |
Common equity ratio, computation of ratio average, number of months |  | 13 months |  |
Common equity ratio |  | 42.00% |  |
Restricted net assets of subsidiaries |  | $ 1,800.0 |  |
Other comprehensive income (loss) |  | 0.0 | 0.0 | 0.0
WPL [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Retained earnings free of dividend restrictions |  | 140.0 |  |
Restricted net assets of subsidiaries |  | 1,700.0 |  |
Other comprehensive income (loss) |  | 0.0 | $ 0.0 | 0.0
At The Market Offering Program [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Maximum aggregate gross sales price of common stock that can be offered and sold |  | $ 125.0 |  | $ 150.0
Common stock issued during the period, At-the-market offering programs (in shares) |  | 3,074,931 | 0 | 4,373,234
Proceeds from issuance of common stock, net |  | $ 124.0 |  | $ 133.0
Fees and commissions from issuance of common stock |  | $ 1.0 |  | $ 2.0
Scenario, Forecast [Member] | WPL [Member] |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |
Retained earnings free of dividend restrictions | $ 140.0 |  |  |
Common equity ratio, computation of ratio average, number of months | 13 months |  |  |
Common equity ratio | 51.00% |  |  |

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