Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended				12 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating revenues	$ 135,135		$ 140,099		$ 474,145		$ 435,624
Depreciation and amortization	(10,376)		(9,710)		(30,191)		(28,864)
Other operating expenses	(84,639)		(90,489)		(335,422)		(304,735)
Operating Income	40,120		39,900		108,532		102,025
Other income (deductions), net	2,636		3,750		10,736		9,770
Interest (expense) income, net	(5,141)		(4,605)		(14,550)		(13,937)
Income before income taxes	37,615		39,045		104,718		97,858
Income tax (provision) benefit	(14,286)		(14,692)		(39,585)		(37,030)
Net Income (Loss)	23,329		24,353		65,133		60,828
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,619,896				1,619,896				1,579,060
Capital expenditures					66,138		92,092		119,047
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	112,869		119,836		308,418		310,176
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	21,404		18,864		163,335		121,373
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	862		1,399		2,392		4,075
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	135,135		140,099		474,145		435,624
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	113,025		120,008		308,819		310,586
Depreciation and amortization	(6,931)		(6,425)		(19,909)		(19,139)
Other operating expenses	(74,573)		(82,414)		(229,286)		(235,390)
Operating Income	31,521		31,169		59,624		56,057
Other income (deductions), net	136		1,166		2,999		2,226
Interest (expense) income, net	(2,832)		(2,309)		(7,571)		(7,028)
Income before income taxes	28,825		30,026		55,052		51,255
Income tax (provision) benefit	(10,758)		(11,021)		(19,602)		(18,148)
Net Income (Loss)	18,067		19,005		35,450		33,107
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	914,839				914,839				899,257
Capital expenditures					50,033				100,146
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	23,018		21,621		170,465		131,822
Depreciation and amortization	(1,575)		(1,497)		(4,700)		(4,357)
Other operating expenses	(22,199)		(21,485)		(144,907)		(111,494)
Operating Income	(756)		(1,361)		20,858		15,971
Other income (deductions), net	8		92		(44)		69
Interest (expense) income, net	(808)		(729)		(2,429)		(2,166)
Income before income taxes	(1,556)		(1,998)		18,385		13,874
Income tax (provision) benefit	651		751		(7,378)		(5,844)
Net Income (Loss)	(905)		(1,247)		11,007		8,030
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	263,688				263,688				265,694
Capital expenditures					14,599				15,554
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	11,536		12,059		34,409		35,999
Depreciation and amortization	(1,858)		(1,788)		(5,547)		(5,368)
Other operating expenses	(35)		(33)		(105)		(96)
Operating Income	9,643		10,238		28,757		30,535
Other income (deductions), net	0		0		0		0
Interest (expense) income, net	(1,545)		(1,596)		(4,676)		(4,814)
Income before income taxes	8,098		8,642		24,081		25,721
Income tax (provision) benefit	(3,251)		(3,469)		(9,665)		(10,209)
Net Income (Loss)	4,847		5,173		14,416		15,512
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	307,599				307,599				288,116
Capital expenditures					1,506				3,347
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		(1)		0		(1)
Operating Income	0		(1)		0		(1)
Other income (deductions), net	2,423		2,340		7,306		7,045
Interest (expense) income, net	0		0		0		0
Income before income taxes	2,423		2,339		7,306		7,044
Income tax (provision) benefit	(963)		(939)		(2,924)		(2,837)
Net Income (Loss)	1,460		1,400		4,382		4,207
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	67,476				67,476				64,504
Capital expenditures					0				0
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues	0		0		0		0
Depreciation and amortization	(12)		0		(35)		0
Other operating expenses	(276)		(145)		(672)		(537)
Operating Income	(288)		(145)		(707)		(537)
Other income (deductions), net	69		152		475		430
Interest (expense) income, net	44		29		126		71
Income before income taxes	(175)		36		(106)		(36)
Income tax (provision) benefit	35		(14)		(16)		8
Net Income (Loss)	(140)		22		(122)		(28)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	443,898				443,898				431,436
Capital expenditures					0				0
Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	30,962				30,962				19,853
Capital expenditures					0				0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(12,444)		(13,589)		(39,548)		(42,783)
Depreciation and amortization	0		0		0		0
Other operating expenses	12,444		13,589		39,548		42,783
Operating Income	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 (Loss)	0		0		0		0
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(408,566)				(408,566)				(389,800)
Capital expenditures					0				0
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	156		172		401		410
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,614		2,757		7,130		10,449
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,674		10,660		32,017		31,924
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	135,147		140,099		474,178		435,624
Depreciation and amortization	(10,364)		(9,710)		(30,156)		(28,864)
Other operating expenses	(97,712)	[1]	(104,143)	[1]	(370,962)	[1]	(338,435)	[1]
Operating Income	27,071		26,246		73,060		68,325
Other income (deductions), net	1,583		2,719		6,871		6,539
Interest (expense) income, net	(5,185)		(4,634)		(14,676)		(14,008)
Income tax (provision) benefit	(13,338)		(13,798)		(36,180)		(34,237)
Net Income Including Noncontrolling Interest	23,469		24,331		65,255		60,856
Less Net Income Attributable to Noncontrolling Interest, net of tax	(6,653)		(6,861)		(19,784)		(20,547)
Net Income (Loss)	16,816		17,470		45,471		40,309
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,558,702				1,558,702				1,530,643
Capital expenditures					66,138		92,092		119,047
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	112,875		119,836		308,435		310,176
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	21,410		18,864		163,351		121,373
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	862		1,399		2,392		4,075
MGE [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	0		0		0		0
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	135,147		140,099		474,178		435,624
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	113,025		120,008		308,819		310,586
Depreciation and amortization	(6,931)		(6,425)		(19,909)		(19,139)
Other operating expenses	(85,314)	[1]	(93,481)	[1]	(248,525)	[1]	(253,566)	[1]
Operating Income	20,780	[1]	20,102	[1]	40,385	[1]	37,881	[1]
Other income (deductions), net	119	[1]	1,212	[1]	2,636	[1]	2,254	[1]
Interest (expense) income, net	(2,832)		(2,309)		(7,571)		(7,028)
Net Income Including Noncontrolling Interest	18,067		19,005		35,450		33,107
Less Net Income Attributable to Noncontrolling Interest, net of tax	0		0		0		0
Net Income (Loss)	18,067		19,005		35,450		33,107
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	917,503				917,503				899,257
Capital expenditures					50,033				100,146
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	23,018		21,621		170,465		131,822
Depreciation and amortization	(1,575)		(1,497)		(4,700)		(4,357)
Other operating expenses	(21,544)	[1]	(20,748)	[1]	(152,182)	[1]	(117,346)	[1]
Operating Income	(101)	[1]	(624)	[1]	13,583	[1]	10,119	[1]
Other income (deductions), net	4	[1]	106	[1]	(147)	[1]	77	[1]
Interest (expense) income, net	(808)		(729)		(2,429)		(2,166)
Net Income Including Noncontrolling Interest	(905)		(1,247)		11,007		8,030
Less Net Income Attributable to Noncontrolling Interest, net of tax	0		0		0		0
Net Income (Loss)	(905)		(1,247)		11,007		8,030
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	262,237				262,237				265,694
Capital expenditures					14,599				15,554
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	11,536		12,059		34,409		35,999
Depreciation and amortization	(1,858)		(1,788)		(5,547)		(5,368)
Other operating expenses	(3,286)	[1]	(3,502)	[1]	(9,770)	[1]	(10,305)	[1]
Operating Income	6,392	[1]	6,769	[1]	19,092	[1]	20,326	[1]
Other income (deductions), net	0	[1]	0	[1]	0	[1]	0	[1]
Interest (expense) income, net	(1,545)		(1,596)		(4,676)		(4,814)
Net Income Including Noncontrolling Interest	4,847		5,173		14,416		15,512
Less Net Income Attributable to Noncontrolling Interest, net of tax	0		0		0		0
Net Income (Loss)	4,847		5,173		14,416		15,512
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	286,600				286,600				288,066
Capital expenditures					1,506				3,347
MGE [Member] | 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	[1]	(1)	[1]	0	[1]	(1)	[1]
Operating Income	0	[1]	(1)	[1]	0	[1]	(1)	[1]
Other income (deductions), net	1,460	[1]	1,401	[1]	4,382	[1]	4,208	[1]
Interest (expense) income, net	0		0		0		0
Net Income Including Noncontrolling Interest	1,460		1,400		4,382		4,207
Less Net Income Attributable to Noncontrolling Interest, net of tax	0		0		0		0
Net Income (Loss)	1,460		1,400		4,382		4,207
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	67,476				67,476				64,504
Capital expenditures					0				0
MGE [Member] | Operating Segments [Member] | Assets Not Allocated [Member]
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	30,778				30,778				19,853
Capital expenditures					0				0
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(12,432)		(13,589)		(39,515)		(42,783)
Depreciation and amortization	0		0		0		0
Other operating expenses	12,432	[1]	13,589	[1]	39,515	[1]	42,783	[1]
Operating Income	0	[1]	0	[1]	0	[1]	0	[1]
Other income (deductions), net	0	[1]	0	[1]	0	[1]	0	[1]
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	(6,653)		(6,861)		(19,784)		(20,547)
Net Income (Loss)	(6,653)		(6,861)		(19,784)		(20,547)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(5,892)				(5,892)				(6,731)
Capital expenditures					0				0
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	150		172		384		410
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,608		2,757		7,114		10,449
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,674		10,660		32,017		31,924
MGE [Member] | Consolidation Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0

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