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Common Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Schedule of Common Equity [Line Items] |  |  |
Shares available for issuance under the Amended and Restated OIP, Shareowner Direct Plan and 401(k) Savings Plan | 7,300,000.0 |  |
Proceeds from issuance of common stock, net | $ 151.2 | $ 0.0 | $ 0.0
Ownership percentage | 15.00% |  |
Initial right entitlement, number of shares of common stock shareholders have the option to purchase (in shares) | 0.5 |  |
Exercise price, rights (in dollars per share) | $ 110.00 |  |
Percentage of exercise price upon change in beneficial ownership | 200.00% |  |
Other comprehensive income (loss) | $ 0.2 | (0.4) | 0.6
IPL [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Retained earnings free of dividend restrictions | $ 554.0 |  |
Common equity ratio, computation of ratio average, number of months | 13 months |  |
Common equity ratio | 42.00% |  |
Other comprehensive income (loss) | $ 0.0 | 0.0 | 0.0
WPL [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Retained earnings free of dividend restrictions | $ 135.0 |  |
Common equity ratio, computation of ratio average, number of months | 13 months |  |
Other comprehensive income (loss) | $ 0.0 | $ 0.0 | $ 0.0
Minimum [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Ownership percentage | 10.00% |  |
2016 [Member] | WPL [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Retained earnings free of dividend restrictions | $ 135.0 |  |
Common equity ratio | 52.25% |  |
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) | 2,186,617 | 0 | 0
Proceeds from issuance of common stock, net | $ 133.0 |  |
Fees and commissions from issuance of common stock | $ 2.0 |  |

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