Taxes (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate	21.00%		35.00%	21.00%		35.00%
State income taxes, net of federal benefit	6.30%		5.20%	6.30%		5.20%
Amortized investment tax credits	(0.10%)		(0.30%)	(0.10%)		(0.30%)
Credit for electricity from wind energy	(0.40%)		(1.50%)	(0.30%)		(1.40%)
Domestic manufacturing deduction	0.00%		(1.90%)	0.00%		(1.70%)
AFUDC equity, net	(1.10%)		(0.30%)	(0.90%)		(0.30%)
Amortization of utility excess deferred tax - tax reform	(1.90%)	[1]	0.00%	(1.90%)	[1]	0.00%
Other, net, individually significant	0.30%		(0.20%)	0.00%		(0.10%)
Effective income tax rate	24.10%		36.00%	24.10%		36.40%
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities - current	$ 9,110			$ 9,110			$ 5,633
Revenue Subject to Refund | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities - current	1,700			1,700
Return of tax credit	4,100			4,100
Revenue Subject to Refund | Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Return of tax credit, total	$ 600			$ 1,200
MGE [Member]
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate	21.00%		35.00%	21.00%		35.00%
State income taxes, net of federal benefit	6.20%		5.10%	6.20%		5.10%
Amortized investment tax credits	(0.10%)		(0.40%)	(0.10%)		(0.40%)
Credit for electricity from wind energy	(0.40%)		(1.60%)	(0.30%)		(1.60%)
Domestic manufacturing deduction	0.00%		(2.00%)	0.00%		(1.80%)
AFUDC equity, net	(1.10%)		(0.30%)	(1.00%)		(0.30%)
Amortization of utility excess deferred tax - tax reform	(1.90%)	[1]	(0.10%)	(2.10%)	[1]	0.00%
Other, net, individually significant	0.20%		(0.20%)	0.00%		(0.10%)
Effective income tax rate	23.90%		35.50%	23.70%		35.90%
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities - current	$ 9,110			$ 9,110			$ 5,633

[1] Included are impacts of the Tax Act for the regulated utility for excess deferred taxes recognized using a normalization method of accounting.