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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  | $ 1,537.9 |  |  | $ 1,497.2
IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  | 777.3 |  |  | 755.4
WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  | 760.6 |  |  | 741.8
2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Interim rate increase, amount |  |  |  |  | $ 102.0
Interim rate increase, percent |  |  |  |  | 7.00%
2016 Test Year Retail Electric [Member] | Alliant Energy and IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Increase in base rates |  | 23.0 |  |  |
Scenario, Forecast [Member] | 2017 Test Year Retail Gas [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Interim rate increase, amount | $ 11.0 |  |  |  |
Interim rate increase, percent | 5.00% |  |  |  |
Subsequent Event [Member] | 2017 Test Year Retail Gas [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Requested rate increase, amount |  |  | $ 20.0 |  |
Requested rate increase, percent |  |  | 8.00% |  |
Subsequent Event [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Approved rate increase, amount |  |  |  | $ 130.0 |
Approved rate increase, percent |  |  |  | 9.00% |
Revenue Subject to Refund [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  | 19.0 |  |  |
Revenue Subject to Refund [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  | 8.0 |  |  |
Revenue Subject to Refund [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  | $ 11.0 |  |  |

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