Income Taxes (Details) - USD ($) $ in Thousands		3 Months Ended												12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income tax provision components [Abstract]
Current payable: Federal														$ 18,622	$ 21,125		$ 16,908
Current payable: State														5,163	5,129		3,287
Net-deferred: Federal														120	(8,346)		17,571
Net-deferred: State														3,629	4,264		4,850
Amortized investment tax credits														(100)	(78)		(103)
Total income tax provision		$ 5,642	$ 9,597	$ 5,828	$ 6,367	$ (13,393)	[1]	$ 15,584		$ 8,736		$ 11,167	[1]	$ 27,434	$ 22,094	[1]	$ 42,513
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate														21.00%	35.00%		35.00%
State income taxes, net of federal benefit														6.30%	5.10%		5.10%
Amortized investment tax credits														(0.10%)	0.00%		(0.10%)
Credit for electricity from wind energy														(0.30%)	(1.60%)		(1.60%)
Domestic manufacturing deduction														0.00%	(1.40%)		(1.30%)
AFUDC equity, net														(0.60%)	(0.20%)		(0.20%)
Federal income tax rate reduction														0.00%	(18.10%)		0.00%
Amortization of utility excess deferred tax - tax reform	[2]													(1.80%)	0.00%		0.00%
Other, net, individually significant														0.10%	(0.30%)		(0.90%)
Effective income tax rate														24.60%	18.50%		36.00%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC																		$ (23,638)	$ (24,781)
Accrued expenses																		(15,385)	(15,135)
Pension and other postretirement benefits																		(34,914)	(32,196)
Deferred tax regulatory account																		(37,121)	(36,124)
Derivatives																		(8,861)	(11,525)
Other deferred tax assets																		(15,137)	(12,790)
Gross deferred income tax assets																		(135,056)	(132,551)
Less valuation allowance																		86	86
Net deferred income tax assets																		(134,970)	(132,465)
Components Of Deferred Tax Liabilities [Abstract]
Property-related																		244,114	238,437
Investment in ATC																		50,771	48,324
Bond transactions																		765	832
Pension and other postretirement benefits																		47,644	42,919
Derivatives																		8,861	11,525
Tax deductible prepayments																		6,014	6,169
Other deferred tax liabilities																		8,753	9,389
Gross deferred income tax liabilities																		366,922	357,595
Deferred income taxes																		231,952	225,130
Effects Of The Tax Cuts And Jobs Act [Abstract]
Increase in Regulatory Liability Tax Cuts and Jobs Act																		179,464	159,786
Decrease in Investment ATC Tax Cuts and Jobs Act																		66,339	60,426
Regulatory liabilities - current																		13,826	5,633
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					1,924							2,487		$ 1,924	$ 2,487		$ 2,528
Additions based on tax positions related to current year														425	552		452
Additions based on tax positions related to prior year														272	19		39
Reductions based on tax positions related to prior years														(672)	(1,134)		(532)
Unrecognized tax benefits, ending balance		1,949				1,924								1,949	1,924		2,487
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance					165							388		165	388		311
Reduction in interest expense on uncertain tax positions														(136)	(312)		(27)
Interest expense on uncertain tax positions														162	89		104
Accrued interest on unrecognized tax benefits, ending balance		191				165								191	165		388
Unrecognized Tax Benefits
Unrecognized tax benefits		$ 1,949			$ 1,924	$ 1,924						$ 2,487		1,924	1,924		2,528	1,949	1,924	$ 2,487
Unrecognized tax benefits permanent differences																		300	100	$ 100
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Decrease in Deferred Tax Liability Tax Cuts And Jobs Act																			176,871
Decrease in Regulatory Asset Tax Cuts And Jobs Act																			4,347
Increase in Regulatory Liability Tax Cuts and Jobs Act	[3]																		130,497
Decrease in Investment ATC Tax Cuts and Jobs Act																			20,375
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act	[4]														21,651
Tax Cuts and Jobs Act [Member] | ATC [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Decrease in Investment ATC Tax Cuts and Jobs Act																			(20,400)
Revenue Subject to Refund | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Regulatory liabilities - current																		3,100
Return of tax credit																		8,200
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														2,800
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal														19,926	21,512		17,521
Current payable: State														5,704	5,316		3,497
Net-deferred: Federal														(2,563)	(11,195)		16,391
Net-deferred: State														2,494	3,435		4,485
Amortized investment tax credits														(100)	(78)		(103)
Total income tax provision														$ 25,461	$ 18,990		$ 41,791
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate														21.00%	35.00%		35.00%
State income taxes, net of federal benefit														6.20%	5.10%		5.10%
Amortized investment tax credits														(0.10%)	0.00%		(0.10%)
Credit for electricity from wind energy														(0.40%)	(1.70%)		(1.70%)
Domestic manufacturing deduction														0.00%	(1.50%)		(1.30%)
AFUDC equity, net														(0.50%)	(0.20%)		(0.20%)
Federal income tax rate reduction														0.00%	(19.30%)		0.00%
Amortization of utility excess deferred tax - tax reform	[2]													(2.00%)	0.00%		0.00%
Other, net, individually significant														0.20%	(0.40%)		(0.90%)
Effective income tax rate														24.40%	17.00%		35.90%
Components Of Deferred Tax Assets [Abstract]
Investment in ATC																		0	0
Accrued expenses																		(15,385)	(15,135)
Pension and other postretirement benefits																		(34,914)	(32,196)
Deferred tax regulatory account																		(37,121)	(36,124)
Derivatives																		(8,861)	(11,525)
Other deferred tax assets																		(15,185)	(12,831)
Gross deferred income tax assets																		(111,466)	(107,811)
Less valuation allowance																		86	86
Net deferred income tax assets																		(111,380)	(107,725)
Components Of Deferred Tax Liabilities [Abstract]
Property-related																		244,114	238,544
Investment in ATC																		0	0
Bond transactions																		765	832
Pension and other postretirement benefits																		47,644	42,919
Derivatives																		8,861	11,525
Tax deductible prepayments																		6,014	6,169
Other deferred tax liabilities																		8,598	9,222
Gross deferred income tax liabilities																		315,996	309,211
Deferred income taxes																		204,616	201,486
Effects Of The Tax Cuts And Jobs Act [Abstract]
Decrease in Investment ATC Tax Cuts and Jobs Act																		0	0
Regulatory liabilities - current																		13,826	5,633
Operating loss deductions and tax credit carryforwards [Abstract]
State net tax operating loss deductions																		$ 1,400
MGE [Member] | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Decrease in Deferred Tax Liability Tax Cuts And Jobs Act																			156,493
Decrease in Regulatory Asset Tax Cuts And Jobs Act																			4,347
Increase in Regulatory Liability Tax Cuts and Jobs Act	[3]																		130,497
Decrease in Investment ATC Tax Cuts and Jobs Act																			$ 0
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act	[4]														$ 21,649

[1] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act. See Footnote 12 for further information.

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

[3] Given that changes in income taxes are generally passed through in customer rates for the regulated utility, a regu latory liability was recorded. The amount and timing of the cash impacts will depend on the period over which certain income tax benefits are provided to customers, which will be subject to review by the PSCW. A portion of the regulatory liability will be returned to customers based on a mandated timeframe dictated by applicable tax laws.

[4] Generated by nonregulated activities.