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Regulatory Matters - Electric Rates (Details) - MPUC [Member] - Minnesota Power [Member] - USD ($) $ in Millions |  |  |  | 3 Months Ended | 4 Months Ended | 18 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Apr. 01, 2021 | May 01, 2020 | Nov. 01, 2019 | Mar. 31, 2021 | Mar. 31, 2020 | Apr. 30, 2020 | Dec. 31, 2019 | Dec. 31, 2020
CIP Consolidated Filing [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Revenue from Cost Recovery Riders and Other Regulatory Programs | $ 2.4 |  |  |  |  |  |  |
Fuel Adjustment Clause Filing [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Forced Outage Costs |  |  |  |  |  |  | $ 8.0 |
COVID-19 Related Deferred Accounting [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Estimated Annual Reduction to Revenue Due to Idled Customers Keetac and Verso |  |  |  | $ 30.0 |  |  |  |
Electric Rates [Member] | Minnesota Cost Recovery Riders [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Revenue from Cost Recovery Riders and Other Regulatory Programs |  |  |  | $ 9.6 | $ 9.6 |  |  |
Electric Rates [Member] | 2020 Minnesota General Rate Case [Member] | Retail Customers [Member] |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Annual Additional Revenue Generated from Requested Final Rate Increase |  |  | $ 66.0 |  |  |  |  |
Requested Rate Increase Percent |  |  | 10.60% |  |  |  |  |
Interim Rate Increase |  |  |  |  |  | $ 36.1 |  |
Requested Return on Equity |  |  | 10.05% |  |  |  |  |
Requested Equity Capital Structure |  |  | 53.81% |  |  |  |  |
Ongoing Customer Rate Increase |  | 4.10% |  |  |  |  |  |
Public Utilities, Interim Rate Increase (Decrease), Percentage |  |  |  |  |  | 5.80% |  |
Public Utilities Reserve for Interim Rates |  |  |  |  |  |  |  | $ 11.7

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