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Regulatory Matters - Regulatory Assets and Liabilities - Footnote (Details) $ in Thousands |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Aug. 01, 2023 USD ($) | Feb. 28, 2021 USD ($) | Sep. 30, 2023 USD ($) | Dec. 31, 2023 USD ($) | Jan. 30, 2024 USD ($) | Dec. 31, 2022 USD ($) | Mar. 01, 2020 MWac
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory asset, amortization period |  |  |  | 26 years |  |  |
Regulatory assets |  |  |  | $ 1,327,683 |  | $ 1,271,501 |
Capital expenditure shortfall refundable to customers (percent) |  |  |  | 80.00% |  |  |
Retroactive rate adjustment collection period |  |  |  | 24 months |  |  |
Regulatory liability, disallowed securitization costs |  |  |  | $ 63,495 |  |  |
Empire Electric |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Public utilities, incurred cost one-time net gain (loss) |  | $ 63,495 | $ 48,452 |  |  |  |
Minimum | Rate adjustment mechanism |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets, recovery period |  |  |  | 1 year |  |  |
Maximum | Rate adjustment mechanism |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets, recovery period |  |  |  | 5 years |  |  |
Fuel and commodity cost adjustments |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Asset retirement obligation |  |  |  |  |  | 21,283 |
Regulatory assets |  |  |  | $ 326,418 |  | 388,294 |
Fuel and commodity cost adjustments | Minimum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory asset, amortization period |  |  |  | 6 months |  |  |
Fuel and commodity cost adjustments | Maximum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory asset, amortization period |  |  |  | 24 months |  |  |
Retired generating plant |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 183,732 |  | 174,609 |
Coal generation capacity (MW) | MWac |  |  |  |  |  |  | 200
Deferred capitalized costs |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory asset, amortization period |  |  |  | 20 years |  |  |
Regulatory assets |  |  |  | $ 124,517 |  | 90,121 |
Capitalized operating and maintenance costs, recovery rate, (percent) |  |  |  | 2.43% |  |  |
Pension and post-employment benefits |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 68,822 |  | 80,736 |
Future service years of employees |  |  |  | 10 years |  |  |
Pension and post-employment benefits | Minimum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets, recovery period |  |  |  | 3 years |  |  |
Pension and post-employment benefits | Maximum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets, recovery period |  |  |  | 8 years |  |  |
Environmental costs |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 66,779 |  | 70,529 |
Environmental remediation, rate recovery period |  |  |  | 7 years |  |  |
Wildfire mitigation and vegetation management |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 64,146 |  | 66,156 |
Regulatory assets, recovery period |  |  |  | 2 years |  |  |
Clean energy and other customer programs |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 37,214 |  | 28,145 |
Clean energy and other customer programs | Minimum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets, recovery period |  |  |  | 6 years |  |  |
Clean energy and other customer programs | Maximum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets, recovery period |  |  |  | 10 years |  |  |
Rate review costs |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 8,815 |  | 9,481 |
Rate review costs | Minimum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Costs capitalized and amortized period |  |  |  | 1 year |  |  |
Rate review costs | Maximum |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Costs capitalized and amortized period |  |  |  | 5 years |  |  |
Long-term maintenance contract |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 4,932 |  | 6,504 |
Regulatory assets, recovery period |  |  |  | 5 years |  |  |
Midwest Extreme Weather Event | Fuel and commodity cost adjustments |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Issuance of securitized tariff bonds, authorized amount |  |  |  |  |  | 221,646 |
Public utilities, incurred cost one-time net gain (loss) | $ 63,495 |  |  |  |  |  |
Public utilities, incurred cost one-time net gain (loss), net of tax |  |  | $ 48,452 |  |  |  |
Energy Transistion Costs | Fuel and commodity cost adjustments |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Issuance of securitized tariff bonds, authorized amount |  |  |  |  |  | $ 140,774 |
Securitized Utility Tariff Bonds due January 1, 2035 | Fuel and commodity cost adjustments | Subsequent Event |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  |  | $ 180,500 |  |
Percentage of public utilities, issuance of securitized tariff bonds |  |  |  |  | 0.04943 |  |
Securitized Utility Tariff Bonds due January 1, 2039 | Fuel and commodity cost adjustments | Subsequent Event |  |  |  |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |  |  |  |
Regulatory assets |  |  |  |  | $ 125,000 |  |
Percentage of public utilities, issuance of securitized tariff bonds |  |  |  |  | 0.05091 |  |

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