Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017		Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 137,795	$ 139,539		$ 419,689	$ 422,825
Depreciation and amortization	(14,259)	(13,372)		(41,754)	(39,606)
Other operating expenses	(83,738)	(84,170)	[1]	(287,718)	(283,791)	[1]
Operating income (loss)	39,798	41,997		90,217	99,428
Other income (deductions), net	4,330	4,943	[1]	13,980	12,038	[1]
Interest (expense) income, net	(5,025)	(4,727)		(14,547)	(14,507)
Income before income taxes	39,103	42,213		89,650	96,959
Income tax (provision) benefit	(9,597)	(15,584)		(21,792)	(35,487)
Net Income (Loss)	29,506	26,629		67,858	61,472
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,974,297			1,974,297			$ 1,855,182
Capital expenditures				149,001	66,286		108,131
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	119,175	120,649		312,633	321,279
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	18,407	18,778		106,152	101,285
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	213	112		904	261
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	137,795	139,539		419,689	422,825
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	119,155	120,539		312,399	320,945
Depreciation and amortization	(9,864)	(9,256)		(28,817)	(27,341)
Other operating expenses	(77,847)	(79,506)	[1]	(228,957)	(233,714)	[1]
Operating income (loss)	31,444	31,777		54,625	59,890
Other income (deductions), net	1,763	2,060	[1]	5,055	3,708	[1]
Interest (expense) income, net	(3,217)	(2,760)		(9,036)	(8,337)
Income before income taxes	29,990	31,077		50,644	55,261
Income tax (provision) benefit	(7,306)	(11,175)		(11,492)	(18,753)
Net Income (Loss)	22,684	19,902		39,152	36,508
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,141,370			1,141,370			1,043,181
Capital expenditures				122,108			77,353
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	22,626	22,670		118,211	112,993
Depreciation and amortization	(2,534)	(2,258)		(7,374)	(6,703)
Other operating expenses	(19,824)	(19,324)	[1]	(99,251)	(93,817)	[1]
Operating income (loss)	268	1,088		11,586	12,473
Other income (deductions), net	829	742	[1]	2,347	1,502	[1]
Interest (expense) income, net	(963)	(795)		(2,689)	(2,399)
Income before income taxes	134	1,035		11,244	11,576
Income tax (provision) benefit	42	(351)		(2,788)	(4,576)
Net Income (Loss)	176	684		8,456	7,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	345,349			345,349			344,337
Capital expenditures				22,406			26,847
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,125	11,242		30,483	33,568
Depreciation and amortization	(1,861)	(1,858)		(5,563)	(5,562)
Other operating expenses	(36)	(50)	[1]	(113)	(153)	[1]
Operating income (loss)	8,228	9,334		24,807	27,853
Other income (deductions), net	0	0	[1]	0	0	[1]
Interest (expense) income, net	(1,320)	(1,371)		(4,002)	(4,170)
Income before income taxes	6,908	7,963		20,805	23,683
Income tax (provision) benefit	(1,852)	(3,196)		(5,638)	(9,505)
Net Income (Loss)	5,056	4,767		15,167	14,178
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	264,735			264,735			270,384
Capital expenditures				4,487			3,931
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	(4)	(9)	[1]	(12)	(9)	[1]
Operating income (loss)	(4)	(9)		(12)	(9)
Other income (deductions), net	1,620	2,258	[1]	5,908	7,242	[1]
Interest (expense) income, net	0	0		0	0
Income before income taxes	1,616	2,249		5,896	7,233
Income tax (provision) benefit	(441)	(906)		(1,611)	(2,909)
Net Income (Loss)	1,175	1,343		4,285	4,324
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	64,722			64,722			61,783
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	(138)	(193)	[1]	(789)	(779)	[1]
Operating income (loss)	(138)	(193)		(789)	(779)
Other income (deductions), net	118	(117)	[1]	670	(414)	[1]
Interest (expense) income, net	475	199		1,180	399
Income before income taxes	455	(111)		1,061	(794)
Income tax (provision) benefit	(40)	44		(263)	256
Net Income (Loss)	415	(67)		798	(538)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	472,740			472,740			485,548
Capital expenditures				0			0
Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	64,016			64,016			26,345
Capital expenditures				0			0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(14,111)	(14,912)		(41,404)	(44,681)
Depreciation and amortization	0	0		0	0
Other operating expenses	14,111	14,912	[1]	41,404	44,681	[1]
Operating income (loss)	0	0		0	0
Other income (deductions), net	0	0	[1]	0	0	[1]
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 (Loss)	0	0		0	0
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(378,635)			(378,635)			(376,396)
Capital expenditures				0			0
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	(20)	(110)		(234)	(334)
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,219	3,892		12,059	11,708
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	9,912	11,130		29,579	33,307
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	137,795	139,539		419,689	422,837
Depreciation and amortization	(14,259)	(13,372)		(41,754)	(39,606)
Other operating expenses	(92,643)	(98,027)	[1],[2]	(306,618)	(315,095)	[1],[2]
Operating income (loss)	30,893	28,140	[2]	71,317	68,136	[2]
Other income (deductions), net	2,523	2,139	[1],[2]	7,185	4,456	[1],[2]
Interest (expense) income, net	(5,500)	(4,926)		(15,727)	(14,906)
Net Income Including Noncontrolling Interest	27,916	25,353		62,775	57,686
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,629)	(5,439)		(16,940)	(16,224)
Net Income (Loss)	22,287	19,914		45,835	41,462
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,814,244			1,814,244			1,684,041
Capital expenditures				149,001	66,286		108,131
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	119,175	120,648		312,633	321,282
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	18,407	18,779		106,152	101,294
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	213	112		904	261
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	137,795	139,539		419,689	422,837
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	119,155	120,539		312,399	320,945
Depreciation and amortization	(9,864)	(9,256)		(28,817)	(27,341)
Other operating expenses	(85,127)	(90,164)	[1],[2]	(240,333)	(251,879)	[1],[2]
Operating income (loss)	24,164	21,119	[2]	43,249	41,725	[2]
Other income (deductions), net	1,737	1,543	[1],[2]	4,939	3,120	[1],[2]
Interest (expense) income, net	(3,217)	(2,760)		(9,036)	(8,337)
Net Income Including Noncontrolling Interest	22,684	19,902		39,152	36,508
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0		0	0
Net Income (Loss)	22,684	19,902		39,152	36,508
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,141,370			1,141,370			1,043,181
Capital expenditures				122,108			77,353
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	22,626	22,670		118,211	112,993
Depreciation and amortization	(2,534)	(2,258)		(7,374)	(6,703)
Other operating expenses	(19,739)	(19,529)	[1],[2]	(101,938)	(98,227)	[1],[2]
Operating income (loss)	353	883	[2]	8,899	8,063	[2]
Other income (deductions), net	786	596	[1],[2]	2,246	1,336	[1],[2]
Interest (expense) income, net	(963)	(795)		(2,689)	(2,399)
Net Income Including Noncontrolling Interest	176	684		8,456	7,000
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0		0	0
Net Income (Loss)	176	684		8,456	7,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	345,349			345,349			344,337
Capital expenditures				22,406			26,847
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,125	11,242		30,483	33,568
Depreciation and amortization	(1,861)	(1,858)		(5,563)	(5,562)
Other operating expenses	(1,888)	(3,246)	[1],[2]	(5,751)	(9,658)	[1],[2]
Operating income (loss)	6,376	6,138	[2]	19,169	18,348	[2]
Other income (deductions), net	0	0	[1],[2]	0	0	[1],[2]
Interest (expense) income, net	(1,320)	(1,371)		(4,002)	(4,170)
Net Income Including Noncontrolling Interest	5,056	4,767		15,167	14,178
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0		0	0
Net Income (Loss)	5,056	4,767		15,167	14,178
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	264,685			264,685			270,334
Capital expenditures				4,487			3,931
MGE [Member] | Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	64,016			64,016			26,345
Capital expenditures				0			0
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(14,111)	(14,912)		(41,404)	(44,669)
Depreciation and amortization	0	0		0	0
Other operating expenses	14,111	14,912	[1],[2]	41,404	44,669	[1],[2]
Operating income (loss)	0	0	[2]	0	0	[2]
Other income (deductions), net	0	0	[1],[2]	0	0	[1],[2]
Interest (expense) income, net	0	0		0	0
Net Income Including Noncontrolling Interest	0	0		0	0
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,629)	(5,439)		(16,940)	(16,224)
Net Income (Loss)	(5,629)	(5,439)		(16,940)	(16,224)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(1,176)			(1,176)			(156)
Capital expenditures				0			$ 0
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	(20)	(109)		(234)	(337)
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,219	3,891		12,059	11,699
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 9,912	$ 11,130		$ 29,579	$ 33,307

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

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