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Regulatory Matters - Electric Rates (Details) - USD ($) $ in Millions |  |  |  |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 02, 2023 | Nov. 01, 2023 | Feb. 28, 2023 | Nov. 01, 2021 | Sep. 30, 2023 | Jun. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | Jun. 30, 2023 | Dec. 31, 2022
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  |  |  |  | $ 443.3 |  | $ 443.3 |  |  | $ 441.0
Regulatory Liabilities |  |  |  |  | 549.3 |  | 549.3 |  |  | 526.1
ECO Annual Filing Member | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Revenue from cost recovery riders and other regulatory programs |  |  |  |  | 2.2 | $ 1.9 |  |  |  |
ECO Triennial Filing | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Conservation improvement program goal, next fiscal year |  |  |  |  |  |  |  |  | $ 12.5 |
Energy conservation and optimization plan goal, second fiscal year |  |  |  |  |  |  |  |  | 12.7 |
Energy conservation and optimization plan goal, third fiscal year |  |  |  |  |  |  |  |  | $ 12.8 |
Electric Rates | Fuel Adjustment Clause 2022 | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  | 13.0
Electric Rates | Fuel Adjustment Clause 2023 | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Regulatory Liabilities |  |  |  |  | 28.3 |  | 28.3 |  |  |
Retail Customers | Electric Rates | Current Cost Recovery Rider | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Revenue from cost recovery riders and other regulatory programs |  |  |  |  |  |  | 44.9 | $ 19.5 |  |
Retail Customers | Electric Rates | 2024 Minnesota General Rate Case | MPUC | Minnesota Power | Subsequent Event |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Requested rate increase percent |  | 12.00% |  |  |  |  |  |  |  |
Requested return on equity |  | 10.30% |  |  |  |  |  |  |  |
Requested equity capital structure |  | 53.00% |  |  |  |  |  |  |  |
Annual additional revenue generated from requested final rate increase | $ 64.0 | $ 89.0 |  |  |  |  |  |  |  |
Annual interim rate increase implementation | 60 days |  |  |  |  |  |  |  |  |
Retail Customers | Electric Rates | 2022 Minnesota General Rate Review | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |
Revenue from cost recovery riders and other regulatory programs |  |  | $ 10.0 |  |  |  |  |  |  |
Requested rate increase percent |  |  |  | 18.00% |  |  |  |  |  |
Requested return on equity |  |  |  | 10.25% |  |  |  |  |  |
Requested equity capital structure |  |  |  | 53.81% |  |  |  |  |  |
Annual additional revenue generated from requested final rate increase |  |  |  | $ 108.0 |  |  |  |  |  |
Authorized return on equity |  |  | 9.65% |  |  |  |  |  |  |
Return on common equity |  |  | 52.50% |  |  |  |  |  |  |
Additional revenue from base rates |  |  | $ 60.0 |  |  |  |  |  |  |
Customer refund |  |  |  |  | $ 39.4 |  | $ 39.4 |  |  | $ 18.4

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