Segment Information (Details) - USD ($)		3 Months Ended												12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues		$ 140,079,000	$ 137,795,000	$ 124,262,000	$ 157,632,000	$ 140,274,000		$ 139,539,000		$ 126,463,000		$ 156,823,000		$ 559,768,000	$ 563,099,000		$ 544,745,000
Depreciation and amortization														(56,412,000)	(53,077,000)		(44,646,000)
Other operating expenses														(389,149,000)	(385,397,000)	[1]	(376,217,000)	[1]
Operating Income (Loss)		23,990,000	39,798,000	24,231,000	26,188,000	25,197,000		41,997,000		25,595,000		31,836,000		114,207,000	124,625,000		123,882,000
Other (deductions) income, net														17,055,000	14,399,000	[1]	14,057,000	[1]
Interest (expense) income, net														(19,609,000)	(19,324,000)		(19,866,000)
Income before income taxes														111,653,000	119,700,000		118,073,000
Income tax (provision) benefit		(5,642,000)	(9,597,000)	(5,828,000)	(6,367,000)	13,393,000	[2]	(15,584,000)	[2]	(8,736,000)	[2]	(11,167,000)	[2]	(27,434,000)	(22,094,000)	[2]	(42,513,000)
Net Income		16,361,000	$ 29,506,000	$ 18,351,000	$ 20,001,000	36,134,000		$ 26,629,000		$ 15,543,000		$ 19,300,000		84,219,000	97,606,000		75,560,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		1,988,618,000				1,855,182,000								1,988,618,000	1,855,182,000		1,801,060,000
Capital expenditures														212,197,000	108,131,000		83,659,000
Effects Of The Tax Cuts And Jobs Act [Abstract]
Investment in ATC		66,339,000				60,426,000								66,339,000	60,426,000
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act	[3]														21,651,000
Investment in ATC						20,375,000									20,375,000
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues														400,882,000	413,926,000		409,006,000
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues														157,767,000	148,825,000		134,543,000
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues														1,119,000	348,000		1,196,000
Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0
All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues														559,768,000	563,099,000		544,745,000
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues														400,593,000	413,416,000		410,918,000
Depreciation and amortization														(38,925,000)	(36,660,000)		(29,122,000)
Other operating expenses														(297,374,000)	(308,100,000)	[1]	(313,649,000)	[1]
Operating Income (Loss)														64,294,000	68,656,000		68,147,000
Other (deductions) income, net														7,294,000	4,539,000	[1]	3,668,000	[1]
Interest (expense) income, net														(12,198,000)	(11,257,000)		(11,147,000)
Income before income taxes														59,390,000	61,938,000		60,668,000
Income tax (provision) benefit														(13,453,000)	(20,547,000)	[2]	(20,115,000)
Net Income														45,937,000	41,391,000		40,553,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		1,193,083,000				1,058,988,000								1,193,083,000	1,058,988,000		1,038,307,800
Capital expenditures														176,399,000	77,353,000		50,699,000
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues														173,843,000	163,799,000		155,921,000
Depreciation and amortization														(10,060,000)	(9,000,000)		(8,128,000)
Other operating expenses														(145,883,000)	(135,029,000)	[1]	(128,721,000)	[1]
Operating Income (Loss)														17,900,000	19,770,000		19,072,000
Other (deductions) income, net														3,132,000	1,912,000	[1]	1,668,000	[1]
Interest (expense) income, net														(3,692,000)	(3,234,000)		(3,223,000)
Income before income taxes														17,340,000	18,448,000		17,517,000
Income tax (provision) benefit														(4,474,000)	(7,303,000)	[2]	(6,894,000)
Net Income														12,866,000	11,145,000		10,623,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		377,005,000				354,875,000								377,005,000	354,875,000		329,538,200
Capital expenditures														30,497,000	26,847,000		29,136,000
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues														40,645,000	44,816,000		45,126,000
Depreciation and amortization														(7,427,000)	(7,417,000)		(7,372,000)
Other operating expenses														(144,000)	(190,000)	[1]	(155,000)	[1]
Operating Income (Loss)														33,074,000	37,209,000		37,599,000
Other (deductions) income, net														0	0	[1]	0	[1]
Interest (expense) income, net														(5,307,000)	(5,533,000)		(5,768,000)
Income before income taxes														27,767,000	31,676,000		31,831,000
Income tax (provision) benefit														(7,534,000)	8,860,000	[2]	(12,775,000)
Net Income														20,233,000	40,536,000		19,056,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		265,301,000				270,384,000								265,301,000	270,384,000		271,277,000
Capital expenditures														5,301,000	3,931,000		3,824,000
Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0
Depreciation and amortization														0	0		0
Other operating expenses														(16,000)	(9,000)	[1]	(17,000)	[1]
Operating Income (Loss)														(16,000)	(9,000)		(17,000)
Other (deductions) income, net														8,602,000	9,844,000	[1]	8,429,000	[1]
Interest (expense) income, net														0	0		0
Income before income taxes														8,586,000	9,835,000		8,412,000
Income tax (provision) benefit														(2,345,000)	(3,954,000)	[2]	(2,836,000)
Net Income														6,241,000	5,881,000		5,576,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		66,366,000				61,783,000	[4]							66,366,000	61,783,000	[4]	74,535,000
Capital expenditures														0	0		0
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0
Depreciation and amortization														0	0		(24,000)
Other operating expenses														(1,045,000)	(1,001,000)	[1]	(895,000)	[1]
Operating Income (Loss)														(1,045,000)	(1,001,000)		(919,000)
Other (deductions) income, net														(1,973,000)	(1,896,000)	[1]	292,000	[1]
Interest (expense) income, net														1,588,000	700,000		272,000
Income before income taxes														(1,430,000)	(2,197,000)		(355,000)
Income tax (provision) benefit														372,000	850,000	[2]	107,000
Net Income														(1,058,000)	(1,347,000)		(248,000)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		465,661,000				485,548,000								465,661,000	485,548,000		465,202,000
Capital expenditures														0	0		0
Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues														(55,313,000)	(58,932,000)		(67,220,000)
Depreciation and amortization														0	0		0
Other operating expenses														55,313,000	58,932,000	[1]	67,220,000	[1]
Operating Income (Loss)														0	0		0
Other (deductions) income, net														0	0	[1]	0	[1]
Interest (expense) income, net														0	0		0
Income before income taxes														0	0		0
Income tax (provision) benefit														0	0	[2]	0
Net Income														0	0		0
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		(378,798,000)				(376,396,000)								(378,798,000)	(376,396,000)		(377,800,000)
Capital expenditures														0	0		0
Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues														(289,000)	(510,000)		1,912,000
Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues														16,076,000	14,974,000		21,378,000
Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues														39,526,000	44,468,000		43,930,000
Consolidation/Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0
Consolidation/Elimination Entries [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0
MGE [Member]
Segment Reporting Information [Line Items]
Operating revenues														559,768,000	563,111,000		544,798,000
Depreciation and amortization														(56,412,000)	(53,077,000)		(44,622,000)
Other operating expenses	[5]													(413,186,000)	(402,678,000)	[1],[2]	(414,973,000)	[1]
Operating Income (Loss)	[5]													90,170,000	107,356,000		85,203,000
Other (deductions) income, net	[5]													10,063,000	5,740,000	[1]	9,527,000	[1]
Interest (expense) income, net														(21,197,000)	(20,024,000)		(20,138,000)
Income tax (provision) benefit														(25,461,000)	(18,990,000)		(41,791,000)
Net Income Including Noncontrolling Interest														79,036,000	93,072,000		74,592,000
Less Net Income Attributable to Noncontrolling Interest, net of tax														(22,552,000)	(43,237,000)		(23,358,000)
Net Income														56,484,000	49,835,000		51,234,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		1,834,891,000				1,684,041,000								1,834,891,000	1,684,041,000		1,638,852,000
Capital expenditures														212,197,000	108,131,000		83,659,000
Effects Of The Tax Cuts And Jobs Act [Abstract]
Investment in ATC		0				0								0	0
MGE [Member] | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act	[3]														21,649,000
Investment in ATC						0									0
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues														400,882,000	413,929,000		409,030,000
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues														157,767,000	148,834,000		134,572,000
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues														1,119,000	348,000		1,196,000
MGE [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0	[6]
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues														559,768,000	563,111,000		544,798,000
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues														400,593,000	413,416,000		410,918,000
Depreciation and amortization														(38,925,000)	(36,660,000)		(29,122,000)
Other operating expenses	[5]													(310,626,000)	(328,093,000)	[1],[2]	(333,632,000)	[1]
Operating Income (Loss)	[5]													51,042,000	48,663,000		48,164,000
Other (deductions) income, net	[5]													7,093,000	3,985,000	[1]	3,536,000	[1]
Interest (expense) income, net														(12,198,000)	(11,257,000)		(11,147,000)
Net Income Including Noncontrolling Interest														45,937,000	41,391,000		40,553,000
Less Net Income Attributable to Noncontrolling Interest, net of tax														0	0		0
Net Income														45,937,000	41,391,000		40,553,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		1,193,083,000				1,058,988,000								1,193,083,000	1,058,988,000		1,038,307,800
Capital expenditures														176,399,000	77,353,000		50,699,000
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues														173,843,000	163,799,000		155,921,000
Depreciation and amortization														(10,060,000)	(9,000,000)		(8,128,000)
Other operating expenses	[5]													(150,195,000)	(142,175,000)	[1],[2]	(135,578,000)	[1]
Operating Income (Loss)	[5]													13,588,000	12,624,000		12,215,000
Other (deductions) income, net	[5]													2,970,000	1,755,000	[1]	1,631,000	[1]
Interest (expense) income, net														(3,692,000)	(3,234,000)		(3,223,000)
Net Income Including Noncontrolling Interest														12,866,000	11,145,000		10,623,000
Less Net Income Attributable to Noncontrolling Interest, net of tax														0	0		0
Net Income														12,866,000	11,145,000		10,623,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		377,005,000				354,875,000								377,005,000	354,875,000		329,538,200
Capital expenditures														30,497,000	26,847,000		29,136,000
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues														40,645,000	44,816,000		45,126,000
Depreciation and amortization														(7,427,000)	(7,417,000)		(7,372,000)
Other operating expenses	[5]													(7,678,000)	8,670,000	[1],[2]	(12,930,000)	[1]
Operating Income (Loss)	[5]													25,540,000	46,069,000		24,824,000
Other (deductions) income, net	[5]													0	0	[1]	0	[1]
Interest (expense) income, net														(5,307,000)	(5,533,000)		(5,768,000)
Net Income Including Noncontrolling Interest														20,233,000	40,536,000		19,056,000
Less Net Income Attributable to Noncontrolling Interest, net of tax														0	0		0
Net Income														20,233,000	40,536,000		19,056,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		265,251,000				270,334,000								265,251,000	270,334,000		271,227,000
Capital expenditures														5,301,000	3,931,000		3,824,000
MGE [Member] | Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues														0	0		0	[6]
Depreciation and amortization														0	0		0	[6]
Other operating expenses	[5]													0	0	[1],[2]	0	[1],[6]
Operating Income (Loss)	[5]													0	0		0	[6]
Other (deductions) income, net	[5]													0	0	[1]	4,360,000	[1],[6]
Interest (expense) income, net														0	0		0	[6]
Net Income Including Noncontrolling Interest														0	0		4,360,000	[6]
Less Net Income Attributable to Noncontrolling Interest, net of tax														0	0		0	[6]
Net Income														0	0		4,360,000	[6]
MGE [Member] | Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues														(55,313,000)	(58,920,000)		(67,167,000)
Depreciation and amortization														0	0		0
Other operating expenses	[5]													55,313,000	58,920,000	[1],[2]	67,167,000	[1]
Operating Income (Loss)	[5]													0	0		0
Other (deductions) income, net	[5]													0	0	[1]	0	[1]
Interest (expense) income, net														0	0		0
Net Income Including Noncontrolling Interest														0	0		0
Less Net Income Attributable to Noncontrolling Interest, net of tax														(22,552,000)	(43,237,000)		(23,358,000)
Net Income														(22,552,000)	(43,237,000)		(23,358,000)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		$ (448,000)				$ (156,000)								(448,000)	(156,000)		(221,000)
Capital expenditures														0	0		0
MGE [Member] | Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues														(289,000)	(513,000)		1,888,000
MGE [Member] | Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues														16,076,000	14,965,000		21,349,000
MGE [Member] | Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues														39,526,000	44,468,000		43,930,000
MGE [Member] | Consolidation/Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues														$ 0	$ 0		$ 0	[6]

[1] Reflects retrospective application of new accounting pronouncement related to pension and other postretirement benefits. See Footnote 2 for further information.

[2] 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.

[3] Generated by nonregulated activities.

[4] In December 2017, there was a $20.4 million one-time tax impact as a result of the Tax Act. See Footnote 12 for further information.

[5] Amounts are shown net of the related tax expense, consistent with the presentation on the MGE Consolidated Statements of Income.

[6] As of July 31, 2016, MGE no longer consolidates MGE Energy's proportionate share of equity earnings in MGE Transco. See Footnote 6 for further information.