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Investments (Narrative) (Details) $ in Millions | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jul. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) MW | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2013
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Carrying value |  | $ 381.4 | $ 326.0 | $ 302.9 |
Proceeds from liquidation of company-owned life insurance policies |  |  | 31.0 |  |
WPL [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
WPL's ownership interest in WPL Transco |  |  |  |  | 100.00%
Net reduction in total equity due to transfer of ATC investment to ATI |  |  | 163.6 |  |
IPL [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Proceeds from liquidation of company-owned life insurance policies |  |  | $ 19.0 |  |
Alliant Energy Finance, LLC [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Ownership interest |  | 50.00% |  |  |
Electric capacity of wind farm (in megawatts) | MW |  | 225 |  |  |
Carrying value |  | $ 98.0 |  |  |
Term Loan Credit Agreement [Member] | Alliant Energy Finance, LLC [Member] |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |
Proceeds from the issuance of debt | $ 95.0 |  |  |  |
Variable-rate term loan credit agreement, period | 364 days |  |  |  |

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