Taxes (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019		Mar. 31, 2018
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%		6.40%
Amortized investment tax credits		(0.10%)		(0.10%)
Credit for electricity from wind energy		(5.90%)	[1]	(0.50%)
AFUDC equity, net		(0.20%)		(0.40%)
Amortization of utility excess deferred tax - tax reform	[2]	(2.40%)		(1.80%)
Other, net, individually significant		0.50%		(0.40%)
Effective income tax rate		19.20%		24.20%
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Excess deferred tax - Income statement effect		$ 0.6		$ 0.5
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		6.20%		6.00%
Amortized investment tax credits		(0.10%)		(0.10%)
Credit for electricity from wind energy		(6.50%)	[1]	(0.50%)
AFUDC equity, net		(0.20%)		(0.40%)
Amortization of utility excess deferred tax - tax reform	[2]	(2.60%)		(2.00%)
Other, net, individually significant		0.50%		(0.60%)
Effective income tax rate		18.30%		23.40%

[1] Saratoga Wind Farm became operational in February 2019.

[2] 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. For the three months ended March 31, 201 9 and 201 8 , MGE recognized $ 0.6 million and $ 0.5 million, respectively. The amount and timing of the cash impacts will depend on the period over which certain income tax benefits are provided t o customers, determined by the PSCW.