Taxes (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate		21.00%	21.00%
State income taxes, net of federal benefit		6.30%	6.30%
Amortized investment tax credits		(1.60%)	(0.10%)
Credit for electricity from wind energy		(7.10%)	(6.90%)
AFUDC equity, net		(0.60%)	(1.60%)
Amortization of utility excess deferred tax - tax reform	[1]	(16.70%)	(2.60%)
Other, net, individually significant		0.00%	0.00%
Effective income tax rate		1.30%	16.10%
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Excess deferred tax - Income statement effect		$ 0.7	$ 0.7
Return of unprotected excess deferred taxes		$ 3.3
MGE [Member]
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate		21.00%	21.00%
State income taxes, net of federal benefit		6.30%	6.30%
Amortized investment tax credits		(1.80%)	(0.10%)
Credit for electricity from wind energy		(7.80%)	(7.50%)
AFUDC equity, net		(0.60%)	(1.70%)
Amortization of utility excess deferred tax - tax reform	[1]	(18.30%)	(2.80%)
Other, net, individually significant		(0.10%)	(0.10%)
Effective income tax rate		(1.30%)	15.10%

[1] 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 three months ended March 31, 2021 and 2020, MGE recognized $0.7 million. Included in the 2021 rate settlement was a one-time return to customers of the electric portion of excess deferred taxes related to the 2017 Tax Act not restricted by IRS normalization rules. For the three months ended March 31, 2021, MGE recognized $3.3 million.