Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 138,198	$ 137,795	$ 427,914	$ 419,689
Depreciation and amortization	(18,193)	(14,259)	(53,423)	(41,754)
Other operating expenses	(81,267)	(83,738)	(285,459)	(287,718)
Operating income (loss)	38,738	39,798	89,032	90,217
Other income (deductions), net	5,204	4,330	15,074	13,980
Interest (expense) income, net	(5,831)	(5,025)	(17,227)	(14,547)
Income before income taxes	38,111	39,103	86,879	89,650
Income tax (provision) benefit	(7,454)	(9,597)	(16,667)	(21,792)
Net Income Including Noncontrolling Interest	30,657	29,506	70,212	67,858
Net Income	30,657	29,506	70,212	67,858
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	2,066,973		2,066,973		$ 1,988,618
Capital expenditures			128,389	149,001	212,197
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	120,607	119,175	314,886	312,633
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	17,377	18,407	112,547	106,152
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	214	213	481	904
Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	138,198	137,795	427,914	419,689
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	120,832	119,155	315,508	312,399
Depreciation and amortization	(13,441)	(9,864)	(39,330)	(28,817)
Other operating expenses	(76,817)	(77,847)	(223,242)	(228,957)
Operating income (loss)	30,574	31,444	52,936	54,625
Other income (deductions), net	1,817	1,763	5,258	5,055
Interest (expense) income, net	(3,762)	(3,217)	(11,253)	(9,036)
Income before income taxes	28,629	29,990	46,941	50,644
Income tax (provision) benefit	(4,948)	(7,306)	(6,036)	(11,492)
Net Income	23,681	22,684	40,905	39,152
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,288,752		1,288,752		1,193,083
Capital expenditures			101,609		176,399
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	23,056	22,626	125,118	118,211
Depreciation and amortization	(2,879)	(2,534)	(8,482)	(7,374)
Other operating expenses	(20,217)	(19,824)	(104,587)	(99,251)
Operating income (loss)	(40)	268	12,049	11,586
Other income (deductions), net	855	829	2,579	2,347
Interest (expense) income, net	(1,059)	(963)	(3,132)	(2,689)
Income before income taxes	(244)	134	11,496	11,244
Income tax (provision) benefit	183	42	(2,848)	(2,788)
Net Income	(61)	176	8,648	8,456
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	379,160		379,160		377,005
Capital expenditures			24,551		30,497
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,233	10,125	30,468	30,483
Depreciation and amortization	(1,873)	(1,861)	(5,611)	(5,563)
Other operating expenses	(33)	(36)	(113)	(113)
Operating income (loss)	8,327	8,228	24,744	24,807
Other income (deductions), net	0	0	0	0
Interest (expense) income, net	(1,274)	(1,320)	(3,838)	(4,002)
Income before income taxes	7,053	6,908	20,906	20,805
Income tax (provision) benefit	(1,922)	(1,852)	(5,695)	(5,638)
Net Income	5,131	5,056	15,211	15,167
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	259,542		259,542		265,301
Capital expenditures			2,229		5,301
Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Depreciation and amortization	0	0	0	0
Other operating expenses	0	(4)	0	(12)
Operating income (loss)	0	(4)	0	(12)
Other income (deductions), net	2,372	1,620	6,808	5,908
Interest (expense) income, net	0	0	0	0
Income before income taxes	2,372	1,616	6,808	5,896
Income tax (provision) benefit	(646)	(441)	(1,856)	(1,611)
Net Income	1,726	1,175	4,952	4,285
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	69,829		69,829		66,366
Capital expenditures			0		0
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Depreciation and amortization	0	0	0	0
Other operating expenses	(123)	(138)	(697)	(789)
Operating income (loss)	(123)	(138)	(697)	(789)
Other income (deductions), net	160	118	429	670
Interest (expense) income, net	264	475	996	1,180
Income before income taxes	301	455	728	1,061
Income tax (provision) benefit	(121)	(40)	(232)	(263)
Net Income	180	415	496	798
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	447,307		447,307		465,661
Capital expenditures			0		0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(15,923)	(14,111)	(43,180)	(41,404)
Depreciation and amortization	0	0	0	0
Other operating expenses	15,923	14,111	43,180	41,404
Operating income (loss)	0	0	0	0
Other income (deductions), net	0	0	0	0
Interest (expense) income, net	0	0	0	0
Income before income taxes	0	0	0	0
Income tax (provision) benefit	0	0	0	0
Net Income	0	0	0	0
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(377,617)		(377,617)		(378,798)
Capital expenditures			0		0
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	225	(20)	622	(234)
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	5,679	4,219	12,571	12,059
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,019	9,912	29,987	29,579
Consolidation Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Consolidation Elimination Entries [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
MGE [Member]
Segment Reporting Information [Line Items]
Operating revenues	138,198	137,795	427,914	419,689
Depreciation and amortization	(18,193)	(14,259)	(53,423)	(41,754)
Other operating expenses	(81,144)	(83,595)	(284,762)	(286,917)
Operating income (loss)	38,861	39,941	89,729	91,018
Other income (deductions), net	2,672	2,592	7,837	7,402
Interest (expense) income, net	(6,095)	(5,500)	(18,223)	(15,727)
Income before income taxes	35,438	37,033	79,343	82,693
Income tax (provision) benefit	(6,687)	(9,117)	(14,579)	(19,918)
Net Income Including Noncontrolling Interest	28,751	27,916	64,764	62,775
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,614)	(5,629)	(16,725)	(16,940)
Net Income	23,137	22,287	48,039	45,835
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,927,027		1,927,027		1,834,891
Capital expenditures			128,389	149,001	212,197
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	120,607	119,175	314,886	312,633
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	17,377	18,407	112,547	106,152
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	214	213	481	904
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	138,198	137,795	427,914	419,689
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	120,832	119,155	315,508	312,399
Depreciation and amortization	(13,441)	(9,864)	(39,330)	(28,817)
Other operating expenses	(76,817)	(77,847)	(223,242)	(228,957)
Operating income (loss)	30,574	31,444	52,936	54,625
Other income (deductions), net	1,817	1,763	5,258	5,055
Interest (expense) income, net	(3,762)	(3,217)	(11,253)	(9,036)
Income before income taxes	28,629	29,990	46,941	50,644
Income tax (provision) benefit	(4,948)	(7,306)	(6,036)	(11,492)
Net Income Including Noncontrolling Interest	23,681	22,684	40,905	39,152
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income	23,681	22,684	40,905	39,152
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,288,752		1,288,752		1,193,083
Capital expenditures			101,609		176,399
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	23,056	22,626	125,118	118,211
Depreciation and amortization	(2,879)	(2,534)	(8,482)	(7,374)
Other operating expenses	(20,217)	(19,824)	(104,587)	(99,251)
Operating income (loss)	(40)	268	12,049	11,586
Other income (deductions), net	855	829	2,579	2,347
Interest (expense) income, net	(1,059)	(963)	(3,132)	(2,689)
Income before income taxes	(244)	134	11,496	11,244
Income tax (provision) benefit	183	42	(2,848)	(2,788)
Net Income Including Noncontrolling Interest	(61)	176	8,648	8,456
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income	(61)	176	8,648	8,456
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	379,160		379,160		377,005
Capital expenditures			24,551		30,497
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,233	10,125	30,468	30,483
Depreciation and amortization	(1,873)	(1,861)	(5,611)	(5,563)
Other operating expenses	(33)	(35)	(113)	(113)
Operating income (loss)	8,327	8,229	24,744	24,807
Other income (deductions), net	0	0	0	0
Interest (expense) income, net	(1,274)	(1,320)	(3,838)	(4,002)
Income before income taxes	7,053	6,909	20,906	20,805
Income tax (provision) benefit	(1,922)	(1,853)	(5,695)	(5,638)
Net Income Including Noncontrolling Interest	5,131	5,056	15,211	15,167
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income	5,131	5,056	15,211	15,167
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	259,492		259,492		265,251
Capital expenditures			2,229		5,301
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(15,923)	(14,111)	(43,180)	(41,404)
Depreciation and amortization	0	0	0	0
Other operating expenses	15,923	14,111	43,180	41,404
Operating income (loss)	0	0	0	0
Other income (deductions), net	0	0	0	0
Interest (expense) income, net	0	0	0	0
Income before income taxes	0	0	0	0
Income tax (provision) benefit	0	0	0	0
Net Income Including Noncontrolling Interest	0	0	0	0
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,614)	(5,629)	(16,725)	(16,940)
Net Income	(5,614)	(5,629)	(16,725)	(16,940)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(377)		(377)		(448)
Capital expenditures			0		$ 0
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	225	(20)	622	(234)
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	5,679	4,219	12,571	12,059
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 10,019	$ 9,912	$ 29,987	$ 29,579