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Common Equity (Narrative) (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2016 USD ($) shares | Jun. 30, 2015 USD ($) | Jun. 30, 2016 USD ($) shares | Jun. 30, 2015 USD ($) shares | Apr. 20, 2016 shares | Dec. 31, 2015 shares
Common Equity [Line Items] |  |  |  |  |  |
Proceeds from issuance of common stock, net |  |  | $ 13.8 | $ 139.5 |  |
Common stock split conversion ratio (2:1) |  |  | 2 |  |  |
Common stock, shares authorized (in shares) | shares | 480,000,000 |  | 480,000,000 |  | 240,000,000 | 480,000,000
Other comprehensive income | $ 0.0 | $ 0.0 | $ 0.0 | 0.0 |  |
IPL [Member] |  |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |  |
Common stock, shares authorized (in shares) | shares | 24,000,000 |  | 24,000,000 |  |  | 24,000,000
Retained earnings free of dividend restrictions | $ 556.0 |  | $ 556.0 |  |  |
Restricted net assets of subsidiaries | 1,500.0 |  | 1,500.0 |  |  |
Capital contributions from parent |  |  | 40.0 | 100.0 |  |
Common stock dividends to parent |  |  | 76.1 |  |  |
Other comprehensive income | $ 0.0 | 0.0 | $ 0.0 | 0.0 |  |
WPL [Member] |  |  |  |  |  |
Common Equity [Line Items] |  |  |  |  |  |
Common stock, shares authorized (in shares) | shares | 18,000,000 |  | 18,000,000 |  |  | 18,000,000
Retained earnings free of dividend restrictions | $ 68.0 |  | $ 68.0 |  |  |
Restricted net assets of subsidiaries | 1,700.0 |  | 1,700.0 |  |  |
Common stock dividends to parent |  |  | 67.5 |  |  |
Other 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) | shares |  |  |  | 4,373,234 |  |
Proceeds from issuance of common stock, net |  |  |  | $ 133.0 |  |
Fees and commissions from issuance of common stock |  |  |  | $ 2.0 |  |

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