Income Taxes (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Income tax provision components [Abstract]
Current payable: Federal										$ 4,179,000		$ 8,017,000		$ 18,622,000
Current payable: State										5,095,000		4,647,000		5,163,000
Net-deferred: Federal										6,181,000		3,510,000		120,000
Net-deferred: State										4,182,000		3,702,000		3,629,000
Amortized investment tax credits										(214,000)		(92,000)		(100,000)
Total income tax provision		$ 3,370,000	$ 7,300,000	$ 3,740,000	$ 5,013,000	$ 3,116,000	$ 7,454,000	$ 3,505,000	$ 5,709,000	$ 19,423,000		$ 19,784,000		$ 27,434,000
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										6.30%		6.20%		6.30%
Amortized investment tax credits										(0.20%)		(0.10%)		(0.10%)
Credit for electricity from wind energy										(6.20%)	[1]	(5.70%)	[1]	(0.30%)
AFUDC equity, net										(1.20%)		(0.30%)		(0.60%)
Amortization of utility excess deferred tax	[2]									(2.00%)		(2.40%)		(1.80%)
Other, net, individually significant										(0.30%)		(0.10%)		0.10%
Effective income tax rate										17.40%		18.60%		24.60%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC		21,688,000				22,576,000				$ 21,688,000		$ 22,576,000
Federal tax credits		19,199,000				0				19,199,000		0
Accrued expenses		11,115,000				15,901,000				11,115,000		15,901,000
Pension and other postretirement benefits		35,446,000				30,968,000				35,446,000		30,968,000
Deferred tax regulatory account		41,318,000				35,493,000				41,318,000		35,493,000
Derivatives		3,852,000				7,351,000				3,852,000		7,351,000
Leases		6,704,000				5,167,000				6,704,000		5,167,000
Other deferred tax assets		14,073,000				12,540,000				14,073,000		12,540,000
Gross deferred income tax assets		153,395,000				129,996,000				153,395,000		129,996,000
Less valuation allowance		(38,000)				(86,000)				(38,000)		(86,000)
Net deferred income tax assets		153,357,000				129,910,000				153,357,000		129,910,000
Components Of Deferred Tax Liabilities [Abstract]
Property-related		257,397,000				245,083,000				257,397,000		245,083,000
Investment in ATC		51,518,000				51,569,000				51,518,000		51,569,000
Bond transactions		595,000				657,000				595,000		657,000
Pension and other postretirement benefits		45,658,000				45,683,000				45,658,000		45,683,000
Derivatives		3,852,000				7,351,000				3,852,000		7,351,000
Tax deductible prepayments		9,059,000				9,078,000				9,059,000		9,078,000
Leases		6,704,000				5,167,000				6,704,000		5,167,000
Other deferred tax liabilities		10,045,000				8,624,000				10,045,000		8,624,000
Gross deferred income tax liabilities		384,828,000				373,212,000				384,828,000		373,212,000
Deferred income taxes		231,471,000				243,302,000				231,471,000		243,302,000
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		19,199,000				0				19,199,000		0
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance					2,093,000				1,949,000	2,093,000		1,949,000		$ 1,924,000
Additions based on tax positions related to current year										796,000		741,000		425,000
Additions based on tax positions related to prior year										0		84,000		272,000
Reductions based on tax positions related to prior years										(608,000)		(681,000)		(672,000)
Unrecognized tax benefits, ending balance		2,281,000				2,093,000				2,281,000		2,093,000		1,949,000
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance					$ 176,000				$ 191,000	176,000		191,000		165,000
Reduction in interest expense on uncertain tax positions										(124,000)		(137,000)		(136,000)
Interest expense on uncertain tax positions										102,000		122,000		162,000
Accrued interest on unrecognized tax benefits, ending balance		154,000				176,000				154,000		176,000		191,000
Unrecognized Tax Benefits
Unrecognized tax benefits permanent differences		0				100,000				0		100,000		300,000
State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
State net operating losses		621,000				1,406,000				621,000		1,406,000
Valuation allowances for state net operating losses		(621,000)				(1,406,000)				(621,000)		(1,406,000)
Federal [Member]
Components Of Deferred Tax Assets [Abstract]
Federal tax credits		19,199,000				0				19,199,000		0
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		19,199,000				0				19,199,000		0
Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Excess Deferred Tax Income Statement Effect										2,200,000		2,600,000		2,100,000
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal										3,716,000		7,616,000		19,926,000
Current payable: State										4,790,000		4,608,000		5,704,000
Net-deferred: Federal										4,756,000		2,242,000		(2,563,000)
Net-deferred: State										3,787,000		3,044,000		2,494,000
Amortized investment tax credits										(214,000)		(92,000)		(100,000)
Total income tax provision										$ 16,835,000		$ 17,418,000		$ 25,461,000
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										6.30%		6.10%		6.20%
Amortized investment tax credits										(0.20%)		(0.10%)		(0.10%)
Credit for electricity from wind energy										(6.80%)	[1]	(6.20%)	[1]	(0.40%)
AFUDC equity, net										(1.40%)		(0.30%)		(0.50%)
Amortization of utility excess deferred tax	[2]									(2.20%)		(2.70%)		(2.00%)
Other, net, individually significant										(0.30%)		(0.10%)		0.20%
Effective income tax rate										16.40%		17.70%		24.40%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC		0				0				$ 0		$ 0
Federal tax credits		20,080,000				0				20,080,000		0
Accrued expenses		11,105,000				15,890,000				11,105,000		15,890,000
Pension and other postretirement benefits		35,446,000				30,968,000				35,446,000		30,968,000
Deferred tax regulatory account		41,318,000				35,493,000				41,318,000		35,493,000
Derivatives		3,852,000				7,351,000				3,852,000		7,351,000
Leases		6,704,000				5,167,000				6,704,000		5,167,000
Other deferred tax assets		14,125,000				12,593,000				14,125,000		12,593,000
Gross deferred income tax assets		132,630,000				107,462,000				132,630,000		107,462,000
Less valuation allowance		(38,000)				(86,000)				(38,000)		(86,000)
Net deferred income tax assets		132,592,000				107,376,000				132,592,000		107,376,000
Components Of Deferred Tax Liabilities [Abstract]
Property-related		257,397,000				245,083,000				257,397,000		245,083,000
Investment in ATC		0				0				0		0
Bond transactions		595,000				657,000				595,000		657,000
Pension and other postretirement benefits		45,658,000				45,683,000				45,658,000		45,683,000
Derivatives		3,852,000				7,351,000				3,852,000		7,351,000
Tax deductible prepayments		9,059,000				9,051,000				9,059,000		9,051,000
Leases		6,704,000				5,167,000				6,704,000		5,167,000
Other deferred tax liabilities		9,717,000				8,425,000				9,717,000		8,425,000
Gross deferred income tax liabilities		332,982,000				321,417,000				332,982,000		321,417,000
Deferred income taxes		200,390,000				214,041,000				200,390,000		214,041,000
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		20,080,000				0				20,080,000		0
State net tax operating loss deductions		600,000								600,000
MGE [Member] | State [Member]
Operating loss deductions and tax credit carryforwards [Abstract]
State net operating losses		621,000				1,406,000				621,000		1,406,000
Valuation allowances for state net operating losses		(621,000)				(1,406,000)				(621,000)		(1,406,000)
MGE [Member] | Federal [Member]
Components Of Deferred Tax Assets [Abstract]
Federal tax credits		20,080,000				0				20,080,000		0
Operating loss deductions and tax credit carryforwards [Abstract]
Federal tax credits		$ 20,080,000				$ 0				20,080,000		0
MGE [Member] | Excess Deferred Taxes, Tax Cuts And Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Excess Deferred Tax Income Statement Effect										$ 2,200,000		$ 2,600,000

[1] Saratoga 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 years ended December 31, 2020, 2019, and 2018, MGE recognized $2.2 million, $2.6 million, and $2.1 million, respectively. 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.