Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Income tax provision components [Abstract]
Current payable: Federal										$ 21,125		$ 16,908	$ 16,837
Current payable: State										5,129		3,287	2,774
Net-deferred: Federal										(8,346)		17,571	15,951
Net-deferred: State										4,264		4,850	5,976
Amortized investment tax credits										(78)		(103)	(175)
Total income tax provision	$ (13,393)	[1]	$ 15,584	$ 8,736	$ 11,167	$ 7,570	$ 15,714	$ 9,284	$ 9,945	$ 22,094	[1]	$ 42,513	$ 41,363
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate										35.00%		35.00%	35.00%
State income taxes, net of federal benefit										5.10%		5.10%	5.20%
Amortized investment tax credits										0.00%		(0.10%)	(0.20%)
Credit for electricity from wind energy										(1.60%)		(1.60%)	(1.80%)
Domestic manufacturing deduction										(1.40%)		(1.30%)	(1.40%)
AFUDC equity, net										(0.20%)		(0.20%)	(0.10%)
Federal income tax rate reduction										(18.10%)		0.00%	0.00%
Other, net, individually significant										(0.30%)		(0.90%)	0.00%
Effective income tax rate										18.50%		36.00%	36.70%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related														$ 238,437	$ 344,721
Investment in ATC														48,324	71,898	[2]
Bond transactions														832	1,324
Pension and other postretirement benefits														42,919	63,516
Derivatives														11,525	20,304
Tax deductible prepayments														6,169	8,404
Other deferred tax liabilities														9,389	16,606
Gross deferred income tax liabilities														357,595	526,773
Investment in ATC														(24,781)	(31,212)	[2]
Accrued expenses														(15,135)	(22,410)
Pension and other postretirement benefits														(32,196)	(48,860)
Deferred tax regulatory account														(36,124)	(903)
Derivatives														(11,525)	(20,304)
Other deferred tax assets														(12,790)	(19,340)
Gross deferred income tax assets														(132,551)	(143,029)
Less valuation allowance														86	69
Net deferred income tax assets														(132,465)	(142,960)
Deferred income taxes														225,130	383,813
Effects Of The Tax Cuts And Jobs Act [Abstract]
Investment in ATC														60,426	73,616
Regulatory Liabilities														159,786	29,083
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					2,487				2,528	$ 2,487		$ 2,528	$ 2,365
Additions based on tax positions related to current year										552		452	488
Additions based on tax positions related to prior year										19		39	520
Reductions based on tax positions related to prior years										(1,134)		(532)	(845)
Unrecognized tax benefits, ending balance	1,924					2,487				1,924		2,487	2,528
Interest on unrecognized tax benefits [Roll Forward]
Accrued interest on unrecognized tax benefits, beginning balance					388				311	388		311	92
Reduction in interest expense on uncertain tax positions										(312)		(27)	(102)
Interest expense on uncertain tax positions										89		104	321
Accrued interest on unrecognized tax benefits, ending balance	165					388				165		388	311
Unrecognized Tax Benefits
Unrecognized tax benefits	$ 1,924				$ 2,487	$ 2,487			$ 2,528	2,487		2,487	2,365	1,924	2,487		$ 2,528
Unrecognized tax benefits permanent differences														100	100		$ 0
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Deferred Tax Assets Tax Cuts And Jobs Act														(2,100)
Deferred Tax Liabilities Tax Cuts And Jobs Act														(179,000)
Deferred Income Tax Expense Tax Cuts And Jobs Act										(21,700)
Regulatory Assets Tax Cuts And Jobs Act														(4,300)
Regulatory Liabilities														130,500
Tax Cuts and Jobs Act [Member] | ATC [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Investment in ATC														(20,400)
MGE [Member]
Income tax provision components [Abstract]
Current payable: Federal										21,512		17,521	19,295
Current payable: State										5,316		3,497	3,443
Net-deferred: Federal										(11,195)		16,391	13,538
Net-deferred: State										3,435		4,485	5,305
Amortized investment tax credits										(78)		(103)	(175)
Total income tax provision										$ 18,990		$ 41,791	$ 41,406
Reconciliation of tax provision to statutory federal income tax rate [Abstract]
Statutory federal income tax rate										35.00%		35.00%	35.00%
State income taxes, net of federal benefit										5.10%		5.10%	5.20%
Amortized investment tax credits										0.00%		(0.10%)	(0.20%)
Credit for electricity from wind energy										(1.70%)		(1.70%)	(1.80%)
Domestic manufacturing deduction										(1.50%)		(1.30%)	(1.40%)
AFUDC equity, net										(0.20%)		(0.20%)	(0.10%)
Federal income tax rate reduction										(19.30%)		0.00%	0.00%
Other, net, individually significant										(0.40%)		(0.90%)	0.00%
Effective income tax rate										17.00%		35.90%	36.70%
Deferred tax liabilities (assets) on balance sheet [Abstract]
Property-related														238,544	344,630
Investment in ATC														0	0	[2]
Bond transactions														832	1,324
Pension and other postretirement benefits														42,919	63,516
Derivatives														11,525	20,304
Tax deductible prepayments														6,169	8,404
Other deferred tax liabilities														9,222	16,571
Gross deferred income tax liabilities														309,211	454,749
Investment in ATC														0	0
Accrued expenses														(15,135)	(22,410)
Pension and other postretirement benefits														(32,196)	(48,860)
Deferred tax regulatory account														(36,124)	(903)
Derivatives														(11,525)	(20,304)
Other deferred tax assets														(12,831)	(19,224)
Gross deferred income tax assets														(107,811)	(111,701)
Less valuation allowance														86	69
Net deferred income tax assets														(107,725)	(111,632)
Deferred income taxes														201,486	343,117
Effects Of The Tax Cuts And Jobs Act [Abstract]
Investment in ATC														0	$ 0
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]
Deferred Tax Assets Tax Cuts And Jobs Act														1,400
Deferred Tax Liabilities Tax Cuts And Jobs Act														(155,100)
Deferred Income Tax Expense Tax Cuts And Jobs Act										$ (21,700)
Regulatory Assets Tax Cuts And Jobs Act														(4,300)
Regulatory Liabilities														$ 130,500

[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] As of December 1, 2016, MGE transferred its ownership interest in ATC to MGE Energy, resulting in a deferred intercompany gain and a corresponding step-up in tax basis.