Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017		Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 124,262	$ 126,463		$ 281,894	$ 283,286
Depreciation and amortization	(13,872)	(13,275)		(27,495)	(26,234)
Other operating expenses	(86,159)	(87,593)	[1]	(203,980)	(199,621)	[1]
Operating income (loss)	24,231	25,595		50,419	57,431
Other income (deductions), net	4,731	3,570	[1]	9,650	7,095	[1]
Interest (expense) income, net	(4,783)	(4,886)		(9,522)	(9,780)
Income before income taxes	24,179	24,279		50,547	54,746
Income tax (provision) benefit	(5,828)	(8,736)		(12,195)	(19,903)
Net Income (Loss)	18,351	15,543		38,352	34,843
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,893,445			1,893,445			$ 1,855,182
Capital expenditures				100,389	42,169		108,131
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	99,067	102,306		193,458	200,630
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	24,980	24,081		87,745	82,507
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	215	76		691	149
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	124,262	126,463		281,894	283,286
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	98,822	102,187		193,244	200,406
Depreciation and amortization	(9,575)	(9,185)		(18,953)	(18,085)
Other operating expenses	(74,535)	(78,202)	[1]	(151,110)	(154,208)	[1]
Operating income (loss)	14,712	14,800		23,181	28,113
Other income (deductions), net	1,562	778	[1]	3,292	1,648	[1]
Interest (expense) income, net	(2,952)	(2,801)		(5,819)	(5,577)
Income before income taxes	13,322	12,777		20,654	24,184
Income tax (provision) benefit	(2,986)	(4,124)		(4,186)	(7,578)
Net Income (Loss)	10,336	8,653		16,468	16,606
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,112,411			1,112,411			1,043,181
Capital expenditures				82,148			77,353
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	28,473	27,503		95,585	90,323
Depreciation and amortization	(2,445)	(2,232)		(4,840)	(4,445)
Other operating expenses	(24,232)	(23,522)	[1]	(79,427)	(74,493)	[1]
Operating income (loss)	1,796	1,749		11,318	11,385
Other income (deductions), net	752	361	[1]	1,518	760	[1]
Interest (expense) income, net	(887)	(805)		(1,726)	(1,604)
Income before income taxes	1,661	1,305		11,110	10,541
Income tax (provision) benefit	(372)	(521)		(2,830)	(4,225)
Net Income (Loss)	1,289	784		8,280	6,316
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	333,621			333,621			344,337
Capital expenditures				14,774			26,847
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,062	11,183		20,358	22,326
Depreciation and amortization	(1,852)	(1,858)		(3,702)	(3,704)
Other operating expenses	(34)	(62)	[1]	(77)	(103)	[1]
Operating income (loss)	8,176	9,263		16,579	18,519
Other income (deductions), net	0	0	[1]	0	0	[1]
Interest (expense) income, net	(1,334)	(1,392)		(2,682)	(2,799)
Income before income taxes	6,842	7,871		13,897	15,720
Income tax (provision) benefit	(1,864)	(3,158)		(3,786)	(6,309)
Net Income (Loss)	4,978	4,713		10,111	9,411
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	266,154			266,154			270,384
Capital expenditures				3,467			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	(5)	0	[1]	(8)	0	[1]
Operating income (loss)	(5)	0		(8)	0
Other income (deductions), net	1,916	2,506	[1]	4,288	4,984	[1]
Interest (expense) income, net	0	0		0	0
Income before income taxes	1,911	2,506		4,280	4,984
Income tax (provision) benefit	(522)	(1,006)		(1,170)	(2,003)
Net Income (Loss)	1,389	1,500		3,110	2,981
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	64,135			64,135			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	(448)	(217)	[1]	(651)	(586)	[1]
Operating income (loss)	(448)	(217)		(651)	(586)
Other income (deductions), net	501	(75)	[1]	552	(297)	[1]
Interest (expense) income, net	390	112		705	200
Income before income taxes	443	(180)		606	(683)
Income tax (provision) benefit	(84)	73		(223)	212
Net Income (Loss)	359	(107)		383	(471)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	477,257			477,257			485,548
Capital expenditures				0			0
Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	17,648			17,648			26,345
Capital expenditures				0			0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(13,095)	(14,410)		(27,293)	(29,769)
Depreciation and amortization	0	0		0	0
Other operating expenses	13,095	14,410	[1]	27,293	29,769	[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	(377,781)			(377,781)			(376,396)
Capital expenditures				0			0
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	(245)	(119)		(214)	(224)
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	3,493	3,422		7,840	7,816
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	9,847	11,107		19,667	22,177
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	124,262	126,462		281,894	283,298
Depreciation and amortization	(13,872)	(13,275)		(27,495)	(26,234)
Other operating expenses	(90,836)	(95,131)	[1],[2]	(213,975)	(217,068)	[1],[2]
Operating income (loss)	19,554	18,056	[2]	40,424	39,996	[2]
Other income (deductions), net	2,222	1,092	[1],[2]	4,662	2,317	[1],[2]
Interest (expense) income, net	(5,173)	(4,998)		(10,227)	(9,980)
Net Income Including Noncontrolling Interest	16,603	14,150		34,859	32,333
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,505)	(5,396)		(11,311)	(10,785)
Net Income (Loss)	11,098	8,754		23,548	21,548
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,728,901			1,728,901			1,684,041
Capital expenditures				100,389	42,169		108,131
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	99,067	102,306		193,458	200,634
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	24,980	24,080		87,745	82,515
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	215	76		691	149
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	124,262	126,462		281,894	283,298
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	98,822	102,187		193,244	200,406
Depreciation and amortization	(9,575)	(9,185)		(18,953)	(18,085)
Other operating expenses	(77,475)	(82,289)	[1],[2]	(155,206)	(161,715)	[1],[2]
Operating income (loss)	11,772	10,713	[2]	19,085	20,606	[2]
Other income (deductions), net	1,516	741	[1],[2]	3,202	1,577	[1],[2]
Interest (expense) income, net	(2,952)	(2,801)		(5,819)	(5,577)
Net Income Including Noncontrolling Interest	10,336	8,653		16,468	16,606
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0		0	0
Net Income (Loss)	10,336	8,653		16,468	16,606
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,112,411			1,112,411			1,043,181
Capital expenditures				82,148			77,353
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	28,473	27,503		95,585	90,323
Depreciation and amortization	(2,445)	(2,232)		(4,840)	(4,445)
Other operating expenses	(24,558)	(24,033)	[1],[2]	(82,199)	(78,698)	[1],[2]
Operating income (loss)	1,470	1,238	[2]	8,546	7,180	[2]
Other income (deductions), net	706	351	[1],[2]	1,460	740	[1],[2]
Interest (expense) income, net	(887)	(805)		(1,726)	(1,604)
Net Income Including Noncontrolling Interest	1,289	784		8,280	6,316
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0		0	0
Net Income (Loss)	1,289	784		8,280	6,316
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	333,621			333,621			344,337
Capital expenditures				14,774			26,847
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,062	11,183		20,358	22,326
Depreciation and amortization	(1,852)	(1,858)		(3,702)	(3,704)
Other operating expenses	(1,898)	(3,220)	[1],[2]	(3,863)	(6,412)	[1],[2]
Operating income (loss)	6,312	6,105	[2]	12,793	12,210	[2]
Other income (deductions), net	0	0	[1],[2]	0	0	[1],[2]
Interest (expense) income, net	(1,334)	(1,392)		(2,682)	(2,799)
Net Income Including Noncontrolling Interest	4,978	4,713		10,111	9,411
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0		0	0
Net Income (Loss)	4,978	4,713		10,111	9,411
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	266,104			266,104			270,334
Capital expenditures				3,467			3,931
MGE [Member] | Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	17,648			17,648			26,345
Capital expenditures				0			0
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(13,095)	(14,411)		(27,293)	(29,757)
Depreciation and amortization	0	0		0	0
Other operating expenses	13,095	14,411	[1],[2]	27,293	29,757	[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	(5,505)	(5,396)		(11,311)	(10,785)
Net Income (Loss)	(5,505)	(5,396)		(11,311)	(10,785)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(883)			(883)			(156)
Capital expenditures				0			$ 0
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	(245)	(119)		(214)	(228)
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	3,493	3,423		7,840	7,808
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 9,847	$ 11,107		$ 19,667	$ 22,177

[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.