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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2020 | Dec. 31, 2018
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | $ 1,840.0 |  | $ 1,737.3
Regulatory liabilities |  | 1,451.0 |  | 1,493.2
IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | 1,404.0 |  | 1,279.0
Regulatory liabilities |  | 733.6 |  | $ 754.9
2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Interim rate annual increase, amount |  | 90.0 |  |
Forecast [Member] | 2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Requested rate annual increase, amount |  |  | $ 127.0 |
Billing credit, period |  |  | 12 months |
Forecast [Member] | 2020 Forward-looking Test Period Retail Gas [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Requested rate annual increase, amount |  |  | $ 12.0 |
Tax-related [Member] | 2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 27.0 |  |
Revenue Subject to Refund [Member] | 2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 8.0 |  |
Analog Electric Meters [Member] | Alliant Energy And IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets |  | $ 36.0 |  |
DAEC PPA [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
PPA term amendment |  | 5 years |  |
Recovery period, authorized |  | 5 years |  |
DAEC PPA [Member] | Forecast [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Buyout payment | $ 110.0 |  |  |

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- Definition Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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