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Investments (Narrative) (Details) $ in Millions | 1 Months Ended | 6 Months Ended
--- | --- | ---
Jul. 31, 2017 USD ($) MW | Jun. 30, 2016 USD ($)
Schedule of Investments [Line Items] |  |
Proceeds from liquidation of company-owned life insurance policies |  | $ 31
IPL [Member] |  |
Schedule of Investments [Line Items] |  |
Proceeds from liquidation of company-owned life insurance policies |  | $ 19
Subsequent Event [Member] | Alliant Energy Finance LLC [Member] |  |
Schedule of Investments [Line Items] |  |
Cash equity ownership interest, ownership percentage | 50.00% |
Electric capacity of non-regulated wind farm | MW | 225 |
Term Loan Credit Agreement [Member] | Subsequent Event [Member] | Alliant Energy Finance LLC [Member] |  |
Schedule of Investments [Line Items] |  |
Variable-rate term loan credit agreement | $ 95 |
Variable-rate term loan credit agreement, period | 364 days |
Scenario, Forecast [Member] | Alliant Energy Finance LLC [Member] |  |
Schedule of Investments [Line Items] |  |
Increase in assets from acquisition, subject to working capital adjustments | $ 98 |

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