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Income Taxes (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Income tax provision components [Abstract] |  |  |  |
Current payable: Federal |  | $ 5,887 | $ 4,811 | $ 1,864
Current payable: State |  | 9,093 | 6,875 | 4,637
Net-deferred: Federal |  | 4,335 | (2,695) | 16,317
Net-deferred: State |  | 3,287 | 3,303 | 6,683
Amortized investment tax credits |  | (1,838) | (1,698) | (1,698)
Total income tax provision |  | $ 20,765 | $ 10,596 | $ 27,803
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |
Statutory federal income tax rate |  | 21.00% | 21.00% | 21.00%
State income taxes, net of federal benefit | [1] | 6.30% | 6.30% | 6.20%
Amortized investment tax credits | [2] | (1.10%) | (2.10%) | (0.70%)
Production tax credits |  | (9.70%) | (10.50%) | (5.30%)
AFUDC equity, net | [3] | (0.60%) | (0.70%) | (0.60%)
Amortization of utility excess deferred tax | [4] | (2.70%) | (5.90%) | (1.50%)
Other, net, individually insignificant |  | 0.10% | 0.10% | 0.00%
Effective income tax rate |  | 13.30% | 8.20% | 19.10%
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract] |  |  |  |
Statutory federal income tax rate |  | $ 32,897 | $ 27,544 | $ 30,555
State income taxes, net of federal benefit | [1] | 9,843 | 8,239 | 9,078
Amortized investment tax credits | [2] | (1,697) | (2,775) | (952)
Production tax credits |  | (15,142) | (13,798) | (7,763)
AFUDC equity, net | [3] | (1,033) | (946) | (938)
Other, net, individually insignificant |  | 135 | 90 | 6
Amortization of utility excess deferred tax | [4] | (4,238) | (7,758) | (2,183)
Income Tax Expense (Benefit) |  | 20,765 | 10,596 | 27,803
Components of Deferred Tax Assets [Abstract] |  |  |  |
Investment in ATC |  | 17,657 | 18,467 |
Federal tax credits |  | 13,003 | 21,252 |
Accrued expenses |  | 12,695 | 11,614 |
Pension and other postretirement benefits |  | 1,319 | 6,878 |
Deferred tax regulatory account |  | 38,021 | 37,892 |
Derivatives |  | 477 | 13 |
Leases |  | 7,455 | 7,521 |
Other deferred tax assets |  | 32,342 | 29,290 |
Gross deferred income tax assets |  | 122,969 | 132,927 |
Less valuation allowance |  | (863) | 0 |
Net deferred income tax assets |  | 122,106 | 132,927 |
Components of Deferred Tax Liabilities [Abstract] |  |  |  |
Property-related |  | 346,043 | 334,728 |
Investment in ATC |  | 54,359 | 54,262 |
Bond transactions |  | 288 | 349 |
Pension and other postretirement benefits |  | 22,671 | 26,801 |
Derivatives |  | 477 | 13 |
Tax deductible prepayments |  | 10,280 | 9,677 |
Leases |  | 7,455 | 7,521 |
Other deferred tax liabilities |  | 18,331 | 15,973 |
Gross deferred income tax liabilities |  | 459,904 | 449,324 |
Deferred income taxes, net |  | 337,798 | 316,397 |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |
Federal tax credits |  | 13,003 | 21,252 |
Unrecognized Tax Benefits [Roll Forward] |  |  |  |
Unrecognized tax benefits, beginning balance |  | 1,546 | 1,615 | 2,485
Additions based on tax positions related to current year |  | 461 | 415 | 455
Additions based on tax positions related to prior year |  | 0 | 0 | 0
Reductions based on tax positions related to prior years |  | (444) | (484) | (1,325)
Unrecognized tax benefits, ending balance |  | 1,562 | 1,546 | 1,615
Interest on unrecognized tax benefits [Roll Forward] |  |  |  |
Accrued interest on unrecognized tax benefits, beginning balance |  | 117 | 130 | 189
Reduction in interest expense on uncertain tax positions |  | (65) | (77) | (149)
Interest expense on uncertain tax positions |  | 44 | 64 | 90
Accrued interest on unrecognized tax benefits, ending balance |  | 96 | 117 | 130
Income Taxes Paid, Net [Abstract] |  |  |  |
Income Tax Paid, Federal, after Refund Received, Total | [5] | (3,970) | (16,238) | 4,000
Wisconsin |  |  |  |
Income Taxes Paid, Net [Abstract] |  |  |  |
Income Tax Paid, State and Local, after Refund Received, Total |  | 10,250 | 3,100 | 5,800
Federal |  |  |  |
Components of Deferred Tax Assets [Abstract] |  |  |  |
Federal tax credits |  | 13,003 | 21,252 |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |
Federal tax credits |  | 13,003 | 21,252 |
State and Local Tax Jurisdiction, Other [Member] |  |  |  |
Income Taxes Paid, Net [Abstract] |  |  |  |
Income Tax Paid, State and Local, after Refund Received, Total |  | 66 | 36 | 0
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member] |  |  |  |
Effects Of The Tax Cuts And Jobs Act [Abstract] |  |  |  |
Excess deferred tax - Income statement effect |  | 4,200 | 3,500 | 3,500
Return of unprotected excess deferred taxes |  |  | 4,100 |
Deficient Deferred Taxes, Tax Cuts And Jobs Act [Member] |  |  |  |
Effects Of The Tax Cuts And Jobs Act [Abstract] |  |  |  |
Collection of unprotected of deficient deferred taxes. |  |  |  | 1,300
MGE [Member] |  |  |  |
Income tax provision components [Abstract] |  |  |  |
Current payable: Federal |  | 4,190 | 3,206 | 1,329
Current payable: State |  | 8,538 | 6,309 | 4,560
Net-deferred: Federal |  | 3,567 | (3,342) | 15,345
Net-deferred: State |  | 2,891 | 2,909 | 6,191
Amortized investment tax credits |  | (1,838) | (1,698) | (1,698)
Total income tax provision |  | $ 17,348 | $ 7,384 | $ 25,727
Reconciliation of tax provision to statutory federal income tax rate [Abstract] |  |  |  |
Statutory federal income tax rate |  | 21.00% | 21.00% | 21.00%
State income taxes, net of federal benefit | [1] | 6.20% | 6.20% | 6.20%
Amortized investment tax credits | [2] | (1.20%) | (2.30%) | (0.70%)
Production tax credits |  | (10.50%) | (11.50%) | (5.60%)
AFUDC equity, net | [3] | (0.70%) | (0.80%) | (0.70%)
Amortization of utility excess deferred tax | [4] | (2.90%) | (6.50%) | (1.60%)
Other, net, individually insignificant |  | 0.10% | 0.00% | 0.00%
Effective income tax rate |  | 12.00% | 6.10% | 18.60%
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract] |  |  |  |
Statutory federal income tax rate |  | $ 30,270 | $ 25,133 | $ 29,000
State income taxes, net of federal benefit | [1] | 9,004 | 7,477 | 8,627
Amortized investment tax credits | [2] | (1,698) | (2,775) | (952)
Production tax credits |  | (15,142) | (13,798) | (7,763)
AFUDC equity, net | [3] | (1,033) | (946) | (938)
Other, net, individually insignificant |  | 185 | 51 | (64)
Amortization of utility excess deferred tax | [4] | (4,238) | (7,758) | (2,183)
Income Tax Expense (Benefit) |  | 17,348 | 7,384 | 25,727
Components of Deferred Tax Assets [Abstract] |  |  |  |
Investment in ATC |  | 0 | 0 |
Federal tax credits |  | 13,003 | 21,252 |
Accrued expenses |  | 12,662 | 11,566 |
Pension and other postretirement benefits |  | 1,319 | 6,878 |
Deferred tax regulatory account |  | 38,021 | 37,892 |
Derivatives |  | 477 | 13 |
Leases |  | 7,455 | 7,521 |
Other deferred tax assets |  | 32,202 | 29,048 |
Gross deferred income tax assets |  | 105,139 | 114,170 |
Less valuation allowance |  | (863) | 0 |
Net deferred income tax assets |  | 104,276 | 114,170 |
Components of Deferred Tax Liabilities [Abstract] |  |  |  |
Property-related |  | 346,043 | 334,728 |
Investment in ATC |  | 0 | 0 |
Bond transactions |  | 288 | 349 |
Pension and other postretirement benefits |  | 22,671 | 26,801 |
Derivatives |  | 477 | 13 |
Tax deductible prepayments |  | 10,269 | 9,666 |
Leases |  | 7,455 | 7,521 |
Other deferred tax liabilities |  | 18,271 | 16,053 |
Gross deferred income tax liabilities |  | 405,474 | 395,131 |
Deferred income taxes, net |  | 301,198 | 280,961 |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |
Federal tax credits |  | 13,003 | 21,252 |
Income Taxes Paid, Net [Abstract] |  |  |  |
Income Tax Paid, Federal, after Refund Received, Total | [5] | (5,750) | (18,658) | 3,450
MGE [Member] | Wisconsin |  |  |  |
Income Taxes Paid, Net [Abstract] |  |  |  |
Income Tax Paid, State and Local, after Refund Received, Total |  | 9,650 | 2,450 | 5,990
MGE [Member] | Federal |  |  |  |
Components of Deferred Tax Assets [Abstract] |  |  |  |
Federal tax credits |  | 13,003 | 21,252 |
Operating loss deductions and tax credit carryforwards [Abstract] |  |  |  |
Federal tax credits |  | 13,003 | 21,252 |
MGE [Member] | State and Local Tax Jurisdiction, Other [Member] |  |  |  |
Income Taxes Paid, Net [Abstract] |  |  |  |
Income Tax Paid, State and Local, after Refund Received, Total |  | 0 | 0 | $ 0
Indemnification Agreement [Member] | Renewable Tax Credits Transferred [Member] |  |  |  |
Effects of The Inflation Reduction Act [Abstract] |  |  |  |
Guarantor Obligations, Liquidation Proceeds, Monetary Amount |  | $ 10,900 | $ 18,500 |
[1] | State taxes in Wisconsin made up the majority (greater than 50 percent) of the tax effect in this category.
[2] | MGE accounts for investment tax credits using the deferral method. Under this method, the credits are initially recorded as a liability and amortized as an income tax benefit over the useful life of the related property. In addition, amortization for the year ended December 31, 2025, also reflects additional investment tax credit amortization on transferred credits included in the rate settlement for the period.
[3] | The impact of non-taxable income related to equity portion of the AFUDC, net of depreciation.
[4] | Included are impacts of the 2017 Tax Act for the regulated utility for excess deferred taxes recognized using a normalization method of accounting in recognition of IRS rules that restrict the rate at which the excess deferred taxes may be returned to utility customers. For the years ended December 31, 2025, 2024, and 2023, MGE recognized $4.2 million, $3.5 million, and $3.5 million, respectively. Included in the 2024 rate settlement was a one-time return to customers of excess deferred taxes related to the 2017 Tax Act not restricted by IRS normalization rules. For the year ended December 31, 2024, MGE recognized $4.1 million of deferred taxes not restricted by IRS normalization rules, compared to a net collection from customers of $1.3 million for the year ended December 31, 2023.
[5] | During the years ended December 31, 2025 and December 31, 2024, federal tax payments and receipts included proceeds from the transfer of PTCs under Internal Revenue Code Section 6418. Cash received for income taxes in 2025 includes $10.9 million related to PTCs generated in 2025 that were sold to a third party. Cash received for income taxes in 2024 includes $18.5 million related to PTCs generated in 2024 and 2023 that were sold to a third party.

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