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Investments (Narrative) (Details) $ in Millions | 1 Months Ended | 9 Months Ended
--- | --- | ---
Jul. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) MW | Sep. 30, 2016 USD ($)
Schedule of Investments [Line Items] |  |  |
Proceeds from liquidation of company-owned life insurance policies |  |  | $ 31
Alliant Energy Finance LLC [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Cash equity ownership interest, ownership percentage |  | 50.00% |
Electric capacity of wind farm | MW |  | 225 |
Increase in Other Investments assets from acquisition |  | $ 98 |
IPL [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Proceeds from liquidation of company-owned life insurance policies |  |  | $ 19
Term Loan Credit Agreement [Member] | Alliant Energy Finance LLC [Member] |  |  |
Schedule of Investments [Line Items] |  |  |
Proceeds from issuance of debt | $ 95 |  |
Variable-rate term loan credit agreement, period | 364 days |  |

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