Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017	Dec. 31, 2017
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate	21.00%		35.00%
State income taxes, net of federal benefit	6.40%		5.30%
Amortized investment tax credits	(0.10%)		(0.10%)
Credit for electricity from wind energy	(0.50%)		(1.80%)
Domestic manufacturing deduction	0.00%		(1.30%)
AFUDC equity, net	(0.40%)		(0.20%)
Amortization of utility excess deferred tax - tax reform	(1.80%)	[1]	0.00%
Other, net, individually significant	(0.40%)		(0.20%)
Effective income tax rate	24.20%		36.70%
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities - current	$ 12,325			$ 5,633
Regulatory liabilities	157,890			154,153
Revenue Subject to Refund | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities - current	3,300
Revenue Subject to Refund | Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities	$ 600
MGE [Member]
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate	21.00%		35.00%
State income taxes, net of federal benefit	6.00%		5.10%
Amortized investment tax credits	(0.10%)		(0.10%)
Credit for electricity from wind energy	(0.50%)		(2.00%)
Domestic manufacturing deduction	0.00%		(1.40%)
AFUDC equity, net	(0.40%)		(0.20%)
Amortization of utility excess deferred tax - tax reform	(2.00%)	[1]	0.00%
Other, net, individually significant	(0.60%)		(0.20%)
Effective income tax rate	23.40%		36.20%
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities - current	$ 12,325			5,633
Regulatory liabilities	$ 157,890			$ 154,153

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