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Common Equity (Narrative) (Details) $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2016 USD ($) shares | Mar. 31, 2015 USD ($) | May. 19, 2016 shares | Apr. 20, 2016 shares | Dec. 31, 2015 shares
Common Equity [Line Items] |  |  |  |  |
Common stock split conversion ratio (2:1) | 2 |  |  |  |
Common stock, shares authorized (in shares) | shares | 240,000,000 |  |  |  | 240,000,000
Common stock, shares outstanding (in shares) | shares | 113,562,651 |  |  |  | 113,459,216
Other comprehensive income | $ 0.0 | $ 0.0 |  |  |
IPL [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Common stock, shares authorized (in shares) | shares | 24,000,000 |  |  |  | 24,000,000
Common stock, shares outstanding (in shares) | shares | 13,370,788 |  |  |  | 13,370,788
Retained earnings free of dividend restrictions | $ 562.0 |  |  |  |
Restricted net assets of subsidiaries | 1,500.0 |  |  |  |
Capital contributions from parent | 40.0 | 0.0 |  |  |
Common stock dividends to parent | 38.1 |  |  |  |
Other comprehensive income | $ 0.0 | 0.0 |  |  |
WPL [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Common stock, shares authorized (in shares) | shares | 18,000,000 |  |  |  | 18,000,000
Common stock, shares outstanding (in shares) | shares | 13,236,601 |  |  |  | 13,236,601
Retained earnings free of dividend restrictions | $ 101.0 |  |  |  |
Restricted net assets of subsidiaries | 1,700.0 |  |  |  |
Common stock dividends to parent | 33.8 |  |  |  |
Other comprehensive income | $ 0.0 | $ 0.0 |  |  |
Subsequent Event [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Common stock, shares authorized (in shares) | shares |  |  |  | 480,000,000 |
Scenario, Forecast [Member] |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |
Common stock, shares outstanding (in shares) | shares |  |  | 227,000,000 |  |

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