Taxes (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate		21.00%	21.00%	21.00%	21.00%
State income taxes, net of federal benefit		6.30%	6.30%	6.30%	6.30%
Amortized investment tax credits		(0.10%)	(0.10%)	(0.10%)	(0.10%)
Credit for electricity from wind energy		(5.00%)	(4.80%)	(6.10%)	(5.60%)
AFUDC equity, net		(1.10%)	(0.40%)	(1.30%)	(0.30%)
Amortization of utility excess deferred tax - tax reform	[1]	(2.00%)	(1.90%)	(2.40%)	(2.20%)
Other, net, individually significant		(0.40%)	(0.50%)	(0.10%)	0.10%
Effective income tax rate		18.70%	19.60%	17.30%	19.20%
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	$ 0.6	$ 2.0	$ 1.7
MGE [Member]
Reconciliation of tax provision to statutory federal income tax rate [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%	6.20%
Amortized investment tax credits		(0.10%)	(0.10%)	(0.10%)	(0.10%)
Credit for electricity from wind energy		(5.40%)	(5.20%)	(6.60%)	(6.10%)
AFUDC equity, net		(1.20%)	(0.40%)	(1.40%)	(0.30%)
Amortization of utility excess deferred tax - tax reform	[1]	(2.20%)	(2.10%)	(2.60%)	(2.40%)
Other, net, individually significant		(0.40%)	(0.50%)	0.00%	0.10%
Effective income tax rate		17.90%	18.90%	16.50%	18.40%

[1] Included are impacts of the Tax Cuts and Jobs Act for the regulated utility for excess deferred taxes recognized using a normalization method of accounting. During the three and nine months ended September 30, 2020, MGE recognized $0.7 million and $2.0 million, respectively, compared to $0.6 million and $1.7 million for the comparable periods in 2019. The amount and timing of the cash impacts will depend on the period over which certain income tax benefits are provided to customers, as determined by the PSCW.