Income Taxes (Details) - USD ($)		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income tax provision components [Abstract]
Current payable: Federal		$ (2,589,000)	$ 4,179,000	$ 8,017,000
Current payable: State		3,002,000	5,095,000	4,647,000
Net-deferred: Federal		(1,473,000)	6,181,000	3,510,000
Net-deferred: State		6,310,000	4,182,000	3,702,000
Amortized investment tax credits		(1,135,000)	(214,000)	(92,000)
Total income tax provision		$ 4,115,000	$ 19,423,000	$ 19,784,000
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate		21.00%	21.00%	21.00%
State income taxes, net of federal benefit		6.20%	6.30%	6.20%
Amortized investment tax credits		(1.50%)	(0.20%)	(0.10%)
Credit for electricity from wind energy		(6.00%)	(6.20%)	(5.70%)
AFUDC equity, net		(0.90%)	(1.20%)	(0.30%)
Amortization of utility excess deferred tax	[1]	(14.80%)	(2.00%)	(2.40%)
Other, net, individually significant		(0.30%)	(0.30%)	(0.10%)
Effective income tax rate		3.70%	17.40%	18.60%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC		$ 20,868,000	$ 21,688,000
Federal tax credits		39,161,000	19,199,000
Accrued expenses		11,053,000	11,115,000
Pension and other postretirement benefits		24,888,000	35,446,000
Deferred tax regulatory account		42,401,000	41,318,000
Derivatives		241,000	3,852,000
Leases		7,218,000	6,704,000
Other deferred tax assets		17,390,000	14,073,000
Gross deferred income tax assets		163,220,000	153,395,000
Less valuation allowance		0	(38,000)
Net deferred income tax assets		163,220,000	153,357,000
Components Of Deferred Tax Liabilities [Abstract]
Property-related		274,613,000	257,397,000
Investment in ATC		52,731,000	51,518,000
Bond transactions		534,000	595,000
Pension and other postretirement benefits		34,781,000	45,658,000
Derivatives		241,000	3,852,000
Tax deductible prepayments		10,222,000	9,059,000
Leases		7,218,000	6,704,000
Other deferred tax liabilities		14,029,000	10,045,000
Gross deferred income tax liabilities		394,369,000	384,828,000
Deferred income taxes, net		231,149,000	231,471,000
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		39,161,000	19,199,000
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance		2,281,000	2,093,000	$ 1,949,000
Additions based on tax positions related to current year		714,000	796,000	741,000
Additions based on tax positions related to prior year		0	0	84,000
Reductions based on tax positions related to prior years		(642,000)	(608,000)	(681,000)
Unrecognized tax benefits, ending balance		2,353,000	2,281,000	2,093,000
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance		154,000	176,000	191,000
Reduction in interest expense on uncertain tax positions		(98,000)	(124,000)	(137,000)
Interest expense on uncertain tax positions		94,000	102,000	122,000
Accrued interest on unrecognized tax benefits, ending balance		150,000	154,000	176,000
Unrecognized Tax Benefits
Unrecognized tax benefits permanent differences		0	0	100
State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
State net operating losses		3,000	621,000
Valuation allowances for state net operating losses		(3,000)	(621,000)
Federal [Member]
Components Of Deferred Tax Assets [Abstract]
Federal tax credits		39,161,000	19,199,000
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		39,161,000	19,199,000
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Excess deferred tax - Income statement effect		2,600,000	2,200,000	2,600,000
Return of unprotected excess deferred taxes		13,200,000
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal		(3,434,000)	3,716,000	7,616,000
Current payable: State		3,163,000	4,790,000	4,608,000
Net-deferred: Federal		(1,951,000)	4,756,000	2,242,000
Net-deferred: State		5,605,000	3,787,000	3,044,000
Amortized investment tax credits		(1,135,000)	(214,000)	(92,000)
Total income tax provision		$ 2,248,000	$ 16,835,000	$ 17,418,000
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate		21.00%	21.00%	21.00%
State income taxes, net of federal benefit		6.20%	6.30%	6.10%
Amortized investment tax credits		(1.50%)	(0.20%)	(0.10%)
Credit for electricity from wind energy		(6.40%)	(6.80%)	(6.20%)
AFUDC equity, net		(1.00%)	(1.40%)	(0.30%)
Amortization of utility excess deferred tax	[1]	(15.80%)	(2.20%)	(2.70%)
Other, net, individually significant		(0.30%)	(0.30%)	(0.10%)
Effective income tax rate		2.20%	16.40%	17.70%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC		$ 0	$ 0
Federal tax credits		39,161,000	20,080,000
Accrued expenses		11,047,000	11,105,000
Pension and other postretirement benefits		24,888,000	35,446,000
Deferred tax regulatory account		42,401,000	41,318,000
Derivatives		241,000	3,852,000
Leases		7,218,000	6,704,000
Other deferred tax assets		17,442,000	14,125,000
Gross deferred income tax assets		142,398,000	132,630,000
Less valuation allowance		0	(38,000)
Net deferred income tax assets		142,398,000	132,592,000
Components Of Deferred Tax Liabilities [Abstract]
Property-related		274,613,000	257,397,000
Investment in ATC		0	0
Bond transactions		534,000	595,000
Pension and other postretirement benefits		34,781,000	45,658,000
Derivatives		241,000	3,852,000
Tax deductible prepayments		10,210,000	9,059,000
Leases		7,218,000	6,704,000
Other deferred tax liabilities		13,686,000	9,717,000
Gross deferred income tax liabilities		341,283,000	332,982,000
Deferred income taxes, net		198,885,000	200,390,000
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		39,161,000	20,080,000
MGE [Member] | State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
State net operating losses		3,000	621,000
Valuation allowances for state net operating losses		(3,000)	(621,000)
MGE [Member] | Federal [Member]
Components Of Deferred Tax Assets [Abstract]
Federal tax credits		39,161,000	20,080,000
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		$ 39,161,000	$ 20,080,000

[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 excess deferred taxes may be returned to utility customers. For the years ended December 31, 2021, 2020 and 2019, MGE recognized $2.6 million, $2.2 million, and $2.6 million, respectively. 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 year ended December 31, 2021, MGE recognized $13.2 million.