Taxes (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory federal income tax rate		21.00%	21.00%	21.00%	21.00%
State income taxes, net of federal benefit		6.20%	6.20%	6.20%	6.30%
Amortized investment tax credits		(2.00%)	(0.60%)	(2.20%)	(0.70%)
Credit for electricity from renewable energy		(10.70%)	(4.90%)	(10.80%)	(5.40%)
AFUDC equity, net		(0.60%)	(0.90%)	(0.60%)	(0.60%)
Amortization of utility excess deferred tax - tax reform	[1]	(5.70%)	(1.40%)	(6.10%)	(1.60%)
Other, net, individually insignificant		0.00%	(0.50%)	(0.10%)	(0.20%)
Effective income tax rate		8.20%	18.90%	7.40%	18.80%
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Excess deferred tax - Income statement effect		$ 0.9	$ 0.9	$ 2.6	$ 2.7
Return of unprotected excess deferred taxes		$ 1.0		$ 3.1
Deficient Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Collection of unprotected deficient deferred taxes			$ 0.3		$ 1.0
MGE [Member]
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory federal income tax rate		21.00%	21.00%	21.00%	21.00%
State income taxes, net of federal benefit		6.20%	6.20%	6.20%	6.30%
Amortized investment tax credits		(2.20%)	(0.60%)	(2.40%)	(0.70%)
Credit for electricity from renewable energy		(11.60%)	(5.20%)	(11.70%)	(5.70%)
AFUDC equity, net		(0.60%)	(1.00%)	(0.70%)	(0.70%)
Amortization of utility excess deferred tax - tax reform	[1]	(6.20%)	(1.50%)	(6.70%)	(1.70%)
Other, net, individually insignificant		(0.10%)	(0.50%)	(0.10%)	(0.30%)
Effective income tax rate		6.50%	18.40%	5.60%	18.20%
Indemnification Agreement [Member] | Renewable Tax Credits Transferred [Member]
Effects of The Inflation Reduction Act [Abstract]
Guarantor Obligations, Liquidation Proceeds, Monetary Amount				$ 7.1

[1] Included are impacts of the Tax Cut and Jobs Act of 2017 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 both the three months ended September 30, 2024 and 2023, MGE recognized $0.9 million. For the nine months ended September 30, 2024 and 2023, MGE recognized $2.6 million and $2.7 million, respectively. For the three and nine months ended September 30, 2024, MGE recognized $1.0 million and $3.1 million, respectively, of deferred taxes not restricted by IRS normalization rules, compared to a net collection from customers of $0.3 million and $1.0 million for the three and nine months ended September 30, 2023.