Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income tax provision components [Abstract]
Current payable: Federal									$ 8,017		$ 18,622		$ 21,125
Current payable: State									4,647		5,163		5,129
Net-deferred: Federal									3,510		120		(8,346)
Net-deferred: State									3,702		3,629		4,264
Amortized investment tax credits									(92)		(100)		(78)
Total income tax provision	$ 3,116	$ 7,454	$ 3,505	$ 5,709	$ 5,642	$ 9,597	$ 5,828	$ 6,367	$ 19,784		$ 27,434		$ 22,094	[1]
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate									21.00%		21.00%		35.00%
State income taxes, net of federal benefit									6.20%		6.30%		5.10%
Amortized investment tax credits									(0.10%)		(0.10%)		0.00%
Credit for electricity from wind energy									(5.70%)	[2]	(0.30%)		(1.60%)
Domestic manufacturing deduction									0.00%		0.00%		(1.40%)
AFUDC equity, net									(0.30%)		(0.60%)		(0.20%)
Federal income tax rate reduction									0.00%		0.00%		(18.10%)
Amortization of utility excess deferred tax - tax reform									(2.40%)	[3]	(1.80%)	[3]	0.00%
Other, net, individually significant									(0.10%)		0.10%		(0.30%)
Effective income tax rate									18.60%		24.60%		18.50%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC															$ 22,576	$ 23,638
Accrued expenses															15,901	15,385
Pension and other postretirement benefits															30,968	34,914
Deferred tax regulatory account															35,493	37,121
Derivatives															7,351	8,861
Leases															5,167	0
Other deferred tax assets															12,540	15,137
Gross deferred income tax assets															129,996	135,056
Less valuation allowance															(86)	(86)
Net deferred income tax assets															129,910	134,970
Components Of Deferred Tax Liabilities [Abstract]
Property-related															245,083	244,114
Investment in ATC															51,569	50,771
Bond transactions															657	765
Pension and other postretirement benefits															45,683	47,644
Derivatives															7,351	8,861
Tax deductible prepayments															9,078	6,014
Leases															5,167	0
Other deferred tax liabilities															8,624	8,753
Gross deferred income tax liabilities															373,212	366,922
Deferred income taxes															243,302	231,952
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions															1,400
Unrecognized tax benefits [Roll Forward]
Unrecognized tax benefits, beginning balance				1,949				1,924	$ 1,949		$ 1,924		$ 2,487
Additions based on tax positions related to current year									741		425		552
Additions based on tax positions related to prior year									84		272		19
Reductions based on tax positions related to prior years									(681)		(672)		(1,134)
Unrecognized tax benefits, ending balance	2,093				1,949				2,093		1,949		1,924
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance				191				165	191		165		388
Reduction in interest expense on uncertain tax positions									(137)		(136)		(312)
Interest expense on uncertain tax positions									122		162		89
Accrued interest on unrecognized tax benefits, ending balance	176				191				176		191		165
Unrecognized Tax Benefits
Unrecognized tax benefits	$ 2,093			$ 1,949	$ 1,949			$ 1,924	2,093		1,924		1,924		2,093	1,949	$ 1,924
Unrecognized tax benefits permanent differences															100	300	$ 100
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		2,100
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal									7,616		19,926		21,512
Current payable: State									4,608		5,704		5,316
Net-deferred: Federal									2,242		(2,563)		(11,195)
Net-deferred: State									3,044		2,494		3,435
Amortized investment tax credits									(92)		(100)		(78)
Total income tax provision									$ 17,418		$ 25,461		$ 18,990	[4]
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate									21.00%		21.00%		35.00%
State income taxes, net of federal benefit									6.10%		6.20%		5.10%
Amortized investment tax credits									(0.10%)		(0.10%)		0.00%
Credit for electricity from wind energy									(6.20%)	[2]	(0.40%)		(1.70%)
Domestic manufacturing deduction									0.00%		0.00%		(1.50%)
AFUDC equity, net									(0.30%)		(0.50%)		(0.20%)
Federal income tax rate reduction									0.00%		0.00%		(19.30%)
Amortization of utility excess deferred tax - tax reform									(2.70%)	[3]	(2.00%)	[3]	0.00%
Other, net, individually significant									(0.10%)		0.20%		(0.40%)
Effective income tax rate									17.70%		24.40%		17.00%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC															0	0
Accrued expenses															15,890	15,385
Pension and other postretirement benefits															30,968	34,914
Deferred tax regulatory account															35,493	37,121
Derivatives															7,351	8,861
Leases															5,167	0
Other deferred tax assets															12,593	15,185
Gross deferred income tax assets															107,462	111,466
Less valuation allowance															(86)	(86)
Net deferred income tax assets															107,376	111,380
Components Of Deferred Tax Liabilities [Abstract]
Property-related															245,083	244,114
Investment in ATC															0	0
Bond transactions															657	765
Pension and other postretirement benefits															45,683	47,644
Derivatives															7,351	8,861
Tax deductible prepayments															9,051	6,014
Leases															5,167	0
Other deferred tax liabilities															8,425	8,598
Gross deferred income tax liabilities															321,417	315,996
Deferred income taxes															214,041	$ 204,616
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions															$ 1,400
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									$ 2,600		$ 2,100

[1] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act.

[2] Saratoga became operational in February 2019.

[3] 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, 2019 and 2018, MGE recognized $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.

[4] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act.