Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 145,707	$ 135,135	$ 128,765	$ 210,245	$ 155,263	$ 140,099	$ 128,288	$ 167,237	$ 619,852		$ 590,887		$ 541,323
Depreciation and amortization									(40,695)		(38,838)		(38,707)
Other operating expenses									(441,059)		(424,062)		(389,853)
Operating Income (Loss)	29,566	40,120	24,441	43,971	25,962	39,900	24,497	37,628	138,098		127,987		112,763
Other (deductions) income, net									10,079		10,701		10,069
Interest (expense) income, net									(19,673)		(18,924)		(19,467)
Income before income taxes									128,504		119,764		103,365
Income tax (provision) benefit	(8,600)	(14,286)	(9,034)	(16,265)	(7,829)	(14,692)	(8,660)	(13,678)	(48,185)		(44,859)		(38,919)
Net Income	15,186	23,329	14,087	27,717	14,077	24,353	13,891	22,584	80,319		74,905		64,446
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	1,697,666				1,579,060				1,697,666		1,579,060		1,586,924
Capital expenditures									92,676		119,047		98,435
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									394,849		403,957		392,365
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									221,720		181,462		139,727
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									3,283		5,468		9,231
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									619,852		590,887		541,323
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									395,358		404,494		392,852
Depreciation and amortization									(26,933)		(25,780)		(26,031)
Other operating expenses									(297,409)		(316,277)		(300,149)
Operating Income (Loss)									71,016		62,437		66,672
Other (deductions) income, net									2,847		3,062		1,145
Interest (expense) income, net									(10,410)		(9,645)		(10,189)
Income before income taxes									63,453		55,854		57,628
Income tax (provision) benefit									(22,070)		(19,176)		(20,906)
Net Income									41,383		36,678		36,722
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	948,241				899,257				948,241		899,257		888,444
Capital expenditures									68,067		100,146		81,965
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									230,086		194,091		152,541
Depreciation and amortization									(6,308)		(5,898)		(5,569)
Other operating expenses									(194,203)		(162,661)		(136,567)
Operating Income (Loss)									29,575		25,532		10,405
Other (deductions) income, net									(86)		59		323
Interest (expense) income, net									(3,229)		(2,986)		(2,874)
Income before income taxes									26,260		22,605		7,854
Income tax (provision) benefit									(10,480)		(9,168)		(2,769)
Net Income									15,780		13,437		5,085
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	308,499				265,694				308,499		265,694		285,468
Capital expenditures									22,104		15,554		13,812
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									45,975		48,059		43,945
Depreciation and amortization									(7,407)		(7,156)		(7,107)
Other operating expenses									(139)		(128)		(121)
Operating Income (Loss)									38,429		40,775		36,717
Other (deductions) income, net									0		0		0
Interest (expense) income, net									(6,208)		(6,400)		(6,537)
Income before income taxes									32,221		34,375		30,180
Income tax (provision) benefit									(12,932)		(13,682)		(12,113)
Net Income									19,289		20,693		18,067
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	281,410				288,116				281,410		288,116		323,216
Capital expenditures									2,505		3,347		2,658
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									0		(1)		0
Operating Income (Loss)									0		(1)		0
Other (deductions) income, net									9,150		9,434		9,080
Interest (expense) income, net									0		0		0
Income before income taxes									9,150		9,433		9,080
Income tax (provision) benefit									(3,664)		(3,796)		(3,648)
Net Income									5,486		5,637		5,432
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	67,697				64,504				67,697		64,504		61,064
Capital expenditures									0		0		0
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues									0		0		0
Depreciation and amortization									(47)		(4)		0
Other operating expenses									(875)		(752)		(1,031)
Operating Income (Loss)									(922)		(756)		(1,031)
Other (deductions) income, net									(1,832)		(1,854)		(479)
Interest (expense) income, net									174		107		133
Income before income taxes									(2,580)		(2,503)		(1,377)
Income tax (provision) benefit									961		963		517
Net Income									(1,619)		(1,540)		(860)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	441,109				431,436				441,109		431,436		413,291
Capital expenditures									0		0		0
Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	41,346				19,853				41,346		19,853		18,559
Capital expenditures									0		0		0
Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues									(51,567)		(55,757)		(48,015)
Depreciation and amortization									0		0		0
Other operating expenses									51,567		55,757		48,015
Operating Income (Loss)									0		0		0
Other (deductions) income, net									0		0		0
Interest (expense) income, net									0		0		0
Income before income taxes									0		0		0
Income tax (provision) benefit									0		0		0
Net Income									0		0		0
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	(390,636)				(389,800)				(390,636)		(389,800)		(403,118)
Capital expenditures									0		0		0
Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									509		537		487
Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									8,366		12,629		12,814
Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									42,692		42,591		34,714
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									619,895		590,925		541,323
Depreciation and amortization									(40,648)		(38,834)		(38,707)
Other operating expenses									(485,319)	[1]	(464,868)	[1]	(424,156)	[1]
Operating Income (Loss)									93,928	[1]	87,223	[1]	78,460	[1]
Other (deductions) income, net									7,857	[1]	8,253	[1]	6,446	[1]
Interest (expense) income, net									(19,847)		(19,031)		(19,600)
Income tax (provision) benefit									(49,145)		(45,822)		(39,435)
Net Income Including Noncontrolling Interest									81,938		76,445		65,306
Less Net Income Attributable to Noncontrolling Interest, net of tax									(26,310)		(27,438)		(24,489)
Net Income									55,628		49,007		40,817
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	1,640,622				1,530,643				1,640,622		1,530,643		1,553,651
Capital expenditures									92,676		119,047		98,435
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									394,871		403,980		392,365
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									221,741		181,477		139,727
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									3,283		5,468		9,231
MGE [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues									0		0		0
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									619,895		590,925		541,323
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									395,358		404,494		392,852
Depreciation and amortization									(26,933)		(25,780)		(26,031)
Other operating expenses									(319,175)	[1]	(335,059)	[1]	(320,701)	[1]
Operating Income (Loss)									49,250	[1]	43,655	[1]	46,120	[1]
Other (deductions) income, net									2,543	[1]	2,668	[1]	791	[1]
Interest (expense) income, net									(10,410)		(9,645)		(10,189)
Net Income Including Noncontrolling Interest									41,383		36,678		36,722
Less Net Income Attributable to Noncontrolling Interest, net of tax									0		0		0
Net Income									41,383		36,678		36,722
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	948,241				899,257				948,241		899,257		888,444
Capital expenditures									68,067		100,146		81,965
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									230,086		194,091		152,541
Depreciation and amortization									(6,308)		(5,898)		(5,569)
Other operating expenses									(204,597)	[1]	(171,717)	[1]	(139,236)	[1]
Operating Income (Loss)									19,181	[1]	16,476	[1]	7,736	[1]
Other (deductions) income, net									(172)	[1]	(53)	[1]	223	[1]
Interest (expense) income, net									(3,229)		(2,986)		(2,874)
Net Income Including Noncontrolling Interest									15,780		13,437		5,085
Less Net Income Attributable to Noncontrolling Interest, net of tax									0		0		0
Net Income									15,780		13,437		5,085
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	308,499				265,694				308,499		265,694		285,468
Capital expenditures									22,104		15,554		13,812
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									45,975		48,059		43,945
Depreciation and amortization									(7,407)		(7,156)		(7,107)
Other operating expenses									(13,071)	[1]	(13,810)	[1]	(12,234)	[1]
Operating Income (Loss)									25,497	[1]	27,093	[1]	24,604	[1]
Other (deductions) income, net									0	[1]	0	[1]	0	[1]
Interest (expense) income, net									(6,208)		(6,400)		(6,537)
Net Income Including Noncontrolling Interest									19,289		20,693		18,067
Less Net Income Attributable to Noncontrolling Interest, net of tax									0		0		0
Net Income									19,289		20,693		18,067
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	281,360				288,066				281,360		288,066		323,166
Capital expenditures									2,505		3,347		2,658
MGE [Member] | 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									0	[1]	(1)	[1]	0	[1]
Operating Income (Loss)									0	[1]	(1)	[1]	0	[1]
Other (deductions) income, net									5,486	[1]	5,638	[1]	5,432	[1]
Interest (expense) income, net									0		0		0
Net Income Including Noncontrolling Interest									5,486		5,637		5,432
Less Net Income Attributable to Noncontrolling Interest, net of tax									0		0		0
Net Income									5,486		5,637		5,432
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	67,697				64,504				67,697		64,504		61,064
Capital expenditures									0		0		0
MGE [Member] | Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	41,346				19,853				41,346		19,853		18,559
Capital expenditures									0		0		0
MGE [Member] | Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues									(51,524)		(55,719)		(48,015)
Depreciation and amortization									0		0		0
Other operating expenses									51,524	[1]	55,719	[1]	48,015	[1]
Operating Income (Loss)									0	[1]	0	[1]	0	[1]
Other (deductions) income, net									0	[1]	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									(26,310)		(27,438)		(24,489)
Net Income									(26,310)		(27,438)		(24,489)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	(6,521)				(6,731)				(6,521)		(6,731)		(23,050)
Capital expenditures									0		0		0
MGE [Member] | Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									(487)		514		487
MGE [Member] | Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									8,345		12,614		12,814
MGE [Member] | Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									42,692		42,591		34,714
MGE [Member] | Consolidation/Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues									$ 0		$ 0		$ 0

[1] *Amounts are shown net of the related tax expense, consistent with the presentation on the consolidated MGE Income Statement.