Income Taxes (Details) - USD ($) $ in Thousands		12 Months Ended
		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.