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Regulatory Matters (Narrative) (Details) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) MW | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Regulatory Matters [Line Items] |  |  |  |  |
Electric capacity of wind farm (in megawatts) | MW |  |  | 1,000 |  |
Regulatory liabilities |  |  | $ 1,423.6 | $ 1,493.2 |
IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets not earning a return |  |  | 78.0 |  |
Regulatory liabilities |  |  | 714.7 | 754.9 |
WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets not earning a return |  |  | 6.0 |  |
Regulatory liabilities |  |  | 708.9 | 738.3 |
2020 Test Period Retail Electric [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Interim rate increase (decrease), amount |  |  | 90.0 |  |
2020 Test Period Retail Electric [Member] | IPL [Member] | Tax-related [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  |  | 27.0 |  |
2020 Test Period Retail Electric [Member] | IPL [Member] | Revenue Subject to Refund [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  |  | $ 8.0 |  |
2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  | $ 102.0
Authorized increase (decrease) in final rates, amount |  |  |  | $ 130.0 |
Forward Contracts [Member] | IUB [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Recovery period, authorized |  |  | 10 years |  |
DAEC PPA [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
PPA term amendment |  |  | 5 years |  |
Recovery period, authorized |  |  | 5 years |  |
Forecast [Member] | 2020 Test Period Retail Electric [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  | $ 127.0 |  |  |
Billing credit, period |  | 12 months |  |  |
Forecast [Member] | 2020 Test Period Retail Gas [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  | $ 12.0 |  |  |
Forecast [Member] | DAEC PPA [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Buyout payment | $ 110.0 |  |  |  |

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