Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues		$ 136,509	$ 135,211	$ 117,040	$ 149,873	$ 140,941	$ 138,198	$ 122,147	$ 167,569	$ 538,633	$ 568,855	$ 559,768
Depreciation and amortization										(74,188)	(71,562)	(56,412)
Operating Income (Loss)		18,712	38,325	21,520	31,440	21,877	38,738	19,783	30,512	109,997	110,910	114,207
Interest (expense) income, net										(23,521)	(23,063)	(19,609)
Income before income taxes										111,841	106,658	111,653
Income tax (provision) benefit		(3,370)	(7,300)	(3,740)	(5,013)	(3,116)	(7,454)	(3,505)	(5,709)	(19,423)	(19,784)	(27,434)
Equity earnings of investments										10,221	9,889	8,821
Net Income		15,796	$ 31,794	$ 18,791	$ 26,037	16,662	$ 30,657	$ 15,548	$ 24,007	92,418	86,874	84,219
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		2,253,651				2,081,664				2,253,651	2,081,664	1,988,618
Capital expenditures										203,139	164,036	212,197
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues										393,692	408,285	400,882
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues										144,261	159,875	157,767
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues										680	695	1,119
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										538,633	568,855	559,768
Depreciation and amortization										(74,188)	(71,562)	(56,412)
Operating Income (Loss)										109,997	110,910	114,207
Interest (expense) income, net										(23,521)	(23,063)	(19,609)
Income tax (provision) benefit										(19,423)	(19,784)	(27,434)
Equity earnings of investments										10,221	9,547	8,602
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues										394,457	409,091	400,593
Depreciation and amortization										(54,658)	(52,755)	(38,925)
Operating Income (Loss)										57,847	59,180	64,294
Interest (expense) income, net										(14,446)	(14,978)	(12,198)
Income tax (provision) benefit										(4,230)	(5,037)	(13,453)
Equity earnings of investments										0	0	0
Net Income										50,522	46,318	45,937
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		1,421,302				1,308,277				1,421,302	1,308,277	1,193,083
Capital expenditures										162,210	125,086	176,399
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues										156,418	175,945	173,843
Depreciation and amortization										(12,049)	(11,320)	(10,060)
Operating Income (Loss)										19,674	19,528	17,900
Interest (expense) income, net										(4,370)	(4,237)	(3,692)
Income tax (provision) benefit										(4,805)	(4,753)	(4,474)
Equity earnings of investments										0	0	0
Net Income										14,167	14,088	12,866
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		444,702				408,001				444,702	408,001	377,005
Capital expenditures										36,906	36,193	30,497
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues										41,082	40,715	40,645
Depreciation and amortization										(7,481)	(7,487)	(7,427)
Operating Income (Loss)										33,460	33,084	33,074
Interest (expense) income, net										(4,826)	(5,083)	(5,307)
Income tax (provision) benefit										(7,800)	(7,628)	(7,534)
Equity earnings of investments										0	0	0
Net Income										20,834	20,373	20,233
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		254,298				258,004				254,298	258,004	265,301
Capital expenditures										4,023	2,757	5,301
Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues										0	0	0
Depreciation and amortization										0	0	0
Operating Income (Loss)										(1)	0	(16)
Interest (expense) income, net										0	0	0
Income tax (provision) benefit										(2,786)	(2,602)	(2,345)
Equity earnings of investments										10,221	9,547	8,602
Net Income										7,434	6,945	6,241
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	[1]	74,480				71,668				74,480	71,668	66,366
Capital expenditures	[1]									0	0	0
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues										0	0	0
Depreciation and amortization										0	0	0
Operating Income (Loss)										(983)	(882)	(1,045)
Interest (expense) income, net										121	1,235	1,588
Income tax (provision) benefit										(198)	(236)	(372)
Equity earnings of investments										0	0	0
Net Income										(539)	(850)	(1,058)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		495,483				443,278				495,483	443,278	465,661
Capital expenditures										0	0	0
Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues										(53,324)	(56,896)	(55,313)
Depreciation and amortization										0	0	0
Operating Income (Loss)										0	0	0
Interest (expense) income, net										0	0	0
Income tax (provision) benefit										0	0	0
Equity earnings of investments										0	0	0
Net Income										0	0	0
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		(436,614)				(407,564)				(436,614)	(407,564)	(378,798)
Capital expenditures										0	0	0
Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues										765	806	(289)
Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues										12,157	16,070	16,076
Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues										40,402	40,020	39,526
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										538,633	568,855	559,768
Depreciation and amortization										(74,188)	(71,562)	(56,412)
Operating Income (Loss)										110,981	111,792	115,268
Interest (expense) income, net										(23,642)	(24,298)	(21,197)
Income before income taxes										102,358	98,197	104,497
Income tax (provision) benefit										(16,835)	(17,418)	(25,461)
Net Income										63,130	58,430	56,484
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		2,119,971				1,973,477				2,119,971	1,973,477	1,834,891
Capital expenditures										203,139	164,036	212,197
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues										393,692	408,285	400,882
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues										144,261	159,875	157,767
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues										680	695	1,119
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues										538,633	568,855	559,768
Depreciation and amortization										(74,188)	(71,562)	(56,412)
Operating Income (Loss)										110,981	111,792	115,268
Interest (expense) income, net										(23,642)	(24,298)	(21,197)
Income tax (provision) benefit										(16,835)	(17,418)	(25,461)
Net Income										63,130	58,430	56,484
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues										394,457	409,091	400,593
Depreciation and amortization										(54,658)	(52,755)	(38,925)
Operating Income (Loss)										57,847	59,180	64,294
Interest (expense) income, net										(14,446)	(14,978)	(12,198)
Income tax (provision) benefit										(4,230)	(5,037)	(13,453)
Net Income										50,522	46,318	45,937
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		1,421,302				1,308,277				1,421,302	1,308,277	1,193,083
Capital expenditures										162,210	125,086	176,399
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues										156,418	175,945	173,843
Depreciation and amortization										(12,049)	(11,320)	(10,060)
Operating Income (Loss)										19,674	19,528	17,900
Interest (expense) income, net										(4,370)	(4,237)	(3,692)
Income tax (provision) benefit										(4,805)	(4,753)	(4,474)
Net Income										14,167	14,088	12,866
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		444,702				408,001				444,702	408,001	377,005
Capital expenditures										36,906	36,193	30,497
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues										41,082	40,715	40,645
Depreciation and amortization										(7,481)	(7,487)	(7,427)
Operating Income (Loss)										33,460	33,084	33,074
Interest (expense) income, net										(4,826)	(5,083)	(5,307)
Income tax (provision) benefit										(7,800)	(7,628)	(7,534)
Net Income										20,834	20,373	20,233
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		254,248				257,954				254,248	257,954	265,251
Capital expenditures										4,023	2,757	5,301
MGE [Member] | Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues										(53,324)	(56,896)	(55,313)
Depreciation and amortization										0	0	0
Operating Income (Loss)										0	0	0
Interest (expense) income, net										0	0	0
Income tax (provision) benefit										0	0	0
Net Income										(22,393)	(22,349)	(22,552)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets		$ (281)				$ (755)				(281)	(755)	(448)
Capital expenditures										0	0	0
MGE [Member] | Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues										765	806	(289)
MGE [Member] | Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues										12,157	16,070	16,076
MGE [Member] | Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues										$ 40,402	$ 40,020	$ 39,526

[1] The Transmission Investment segment represents MGE Energy's investment in equity method investees.