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Common Equity (Narrative) (Details) $ / shares in Units, $ in Millions |  | 12 Months Ended
--- | --- | ---
Apr. 20, 2016 shares | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) shares | Dec. 31, 2014 USD ($) shares
Schedule of Common Equity [Line Items] |  |  |  |  |  |  |  |
Common stock split conversion ratio (2:1) | 2 |  |  |  |  |  |  |
Common stock, shares authorized (in shares) | shares | 240,000,000 |  |  | 480,000,000 | [1] | 480,000,000 | [1] |
Shares available for issuance under the Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan (in shares) | shares |  |  |  | 11,300,000.0 |  |  |  |
Proceeds from issuance of common stock, net |  |  |  | $ 26.6 |  | $ 151.2 | [2] | $ 0.0
Ownership percentage |  |  |  | 15.00% |  |  |  |
Initial right entitlement, number of shares of common stock shareholders have the option to purchase (in shares) | shares |  |  |  | 0.25 |  |  |  |
Exercise price, rights (in dollars per share) | $ / shares |  |  |  | $ 55.00 |  |  |  |
Percentage of exercise price upon change in beneficial ownership |  |  |  | 200.00% |  |  |  |
Other comprehensive income (loss) |  |  |  | $ 0.0 |  | $ 0.2 |  | (0.4)
IPL [Member] |  |  |  |  |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) | shares |  |  |  | 24,000,000 |  | 24,000,000 |  |
Retained earnings free of dividend restrictions |  |  |  | $ 618.0 |  |  |  |
Common equity ratio, computation of ratio average, number of months |  |  |  | 13 months |  |  |  |
Common equity ratio |  |  |  | 42.00% |  |  |  |
Restricted net assets of subsidiaries |  |  |  | $ 1,600.0 |  |  |  |
Other comprehensive income (loss) |  |  |  | $ 0.0 |  | $ 0.0 |  | 0.0
WPL [Member] |  |  |  |  |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) | shares |  |  |  | 18,000,000 |  | 18,000,000 |  |
Retained earnings free of dividend restrictions |  |  |  | $ 126.0 |  |  |  |
Common equity ratio, computation of ratio average, number of months |  |  |  | 13 months |  |  |  |
Restricted net assets of subsidiaries |  |  |  | $ 1,600.0 |  |  |  |
Other comprehensive income (loss) |  |  |  | $ 0.0 |  | $ 0.0 |  | $ 0.0
Minimum [Member] |  |  |  |  |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |  |  |  |  |
Ownership percentage |  |  |  | 10.00% |  |  |  |
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 |  |  |  |  |  | $ 150.0 |  |
Common stock issued during the period, At-the-market offering program (in shares) | shares |  |  |  | 0 |  | 4,373,234 |  | 0
Proceeds from issuance of common stock, net |  |  |  |  |  | $ 133.0 |  |
Fees and commissions from issuance of common stock |  |  |  |  |  | $ 2.0 |  |
Scenario, Forecast [Member] | WPL [Member] |  |  |  |  |  |  |  |
Schedule of Common Equity [Line Items] |  |  |  |  |  |  |  |
Retained earnings free of dividend restrictions |  | $ 140.0 | $ 126.0 |  |  |  |  |
Common equity ratio |  | 51.00% | 51.00% |  |  |  |  |
[1] | Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.
[2] | Amounts reflect the effects of a two-for-one common stock split distributed in May 2016.

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