Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Operating revenues		$ 139,539	$ 136,717	$ 422,825	$ 405,820
Depreciation and amortization		(13,372)	(11,212)	(39,606)	(33,358)
Other operating expenses		(83,166)	(78,978)	(280,757)	(268,192)
Operating income (loss)		43,001	46,527	102,462	104,270
Other income (deductions), net		3,939	2,105	9,004	6,726
Interest (expense) income, net		(4,727)	(5,038)	(14,507)	(14,995)
Income before income taxes		42,213	43,594	96,959	96,001
Income tax (provision) benefit		(15,584)	(15,714)	(35,487)	(34,943)
Net Income (Loss)		26,629	27,880	61,472	61,058
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		120,649	118,933	321,279	312,470
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		18,778	17,570	101,285	92,368
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		112	214	261	982
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		139,539	136,717	422,825	405,820
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		120,539	119,392	320,945	313,952
Depreciation and amortization		(9,256)	(7,332)	(27,341)	(21,754)
Other operating expenses		(78,886)	(77,010)	(231,827)	(230,680)
Operating income (loss)		32,397	35,050	61,777	61,518
Other income (deductions), net		1,440	178	1,821	492
Interest (expense) income, net		(2,760)	(2,855)	(8,337)	(8,421)
Income before income taxes		31,077	32,373	55,261	53,589
Income tax (provision) benefit		(11,175)	(11,290)	(18,753)	(17,984)
Net Income (Loss)		19,902	21,083	36,508	35,605
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		22,670	23,610	112,993	109,646
Depreciation and amortization		(2,258)	(2,047)	(6,703)	(6,043)
Other operating expenses		(18,940)	(19,292)	(92,670)	(88,328)
Operating income (loss)		1,472	2,271	13,620	15,275
Other income (deductions), net		358	13	355	(13)
Interest (expense) income, net		(795)	(818)	(2,399)	(2,425)
Income before income taxes		1,035	1,466	11,576	12,837
Income tax (provision) benefit		(351)	(494)	(4,576)	(5,048)
Net Income (Loss)		684	972	7,000	7,789
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		11,242	11,212	33,568	33,876
Depreciation and amortization		(1,858)	(1,833)	(5,562)	(5,537)
Other operating expenses		(50)	(33)	(153)	(115)
Operating income (loss)		9,334	9,346	27,853	28,224
Other income (deductions), net		0	0	0	0
Interest (expense) income, net		(1,371)	(1,436)	(4,170)	(4,348)
Income before income taxes		7,963	7,910	23,683	23,876
Income tax (provision) benefit		(3,196)	(3,175)	(9,505)	(9,583)
Net Income (Loss)		4,767	4,735	14,178	14,293
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		(9)	(13)	(9)	(17)
Operating income (loss)		(9)	(13)	(9)	(17)
Other income (deductions), net		2,258	2,023	7,242	6,023
Interest (expense) income, net		0	0	0	0
Income before income taxes		2,249	2,010	7,233	6,006
Income tax (provision) benefit		(906)	(812)	(2,909)	(2,418)
Net Income (Loss)		1,343	1,198	4,324	3,588
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues		0	0	0	0
Depreciation and amortization		0	0	0	(24)
Other operating expenses		(193)	(127)	(779)	(706)
Operating income (loss)		(193)	(127)	(779)	(730)
Other income (deductions), net		(117)	(109)	(414)	224
Interest (expense) income, net		199	71	399	199
Income before income taxes		(111)	(165)	(794)	(307)
Income tax (provision) benefit		44	57	256	90
Net Income (Loss)		(67)	(108)	(538)	(217)
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues		(14,912)	(17,497)	(44,681)	(51,654)
Depreciation and amortization		0	0	0	0
Other operating expenses		14,912	17,497	44,681	51,654
Operating income (loss)		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
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		(110)	459	(334)	1,482
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		3,892	6,040	11,708	17,278
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		11,130	10,998	33,307	32,894
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		139,539	136,728	422,837	405,860
Depreciation and amortization		(13,372)	(11,212)	(39,606)	(33,334)
Other operating expenses	[1]	(97,023)	(93,781)	(312,061)	(299,998)
Operating income (loss)	[1]	29,144	31,735	71,170	72,528
Other income (deductions), net	[1]	1,135	1,036	1,422	3,615
Interest (expense) income, net		(4,926)	(5,109)	(14,906)	(15,194)
Net Income Including Noncontrolling Interest		25,353	27,662	57,686	60,949
Less Net Income Attributable to Noncontrolling Interest, net of tax		(5,439)	(5,695)	(16,224)	(17,899)
Net Income (Loss)		19,914	21,967	41,462	43,050
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		120,648	118,938	321,282	312,488
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		18,779	17,576	101,294	92,390
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		112	214	261	982
MGE [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	[2]	0	0	0	0
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues		139,539	136,728	422,837	405,860
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		120,539	119,392	320,945	313,952
Depreciation and amortization		(9,256)	(7,332)	(27,341)	(21,754)
Other operating expenses	[1]	(89,544)	(88,282)	(249,992)	(248,566)
Operating income (loss)	[1]	21,739	23,778	43,612	43,632
Other income (deductions), net	[1]	923	160	1,233	394
Interest (expense) income, net		(2,760)	(2,855)	(8,337)	(8,421)
Net Income Including Noncontrolling Interest		19,902	21,083	36,508	35,605
Less Net Income Attributable to Noncontrolling Interest, net of tax		0	0	0	0
Net Income (Loss)		19,902	21,083	36,508	35,605
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		22,670	23,610	112,993	109,646
Depreciation and amortization		(2,258)	(2,047)	(6,703)	(6,043)
Other operating expenses	[1]	(19,145)	(19,781)	(97,080)	(93,348)
Operating income (loss)	[1]	1,267	1,782	9,210	10,255
Other income (deductions), net	[1]	212	8	189	(41)
Interest (expense) income, net		(795)	(818)	(2,399)	(2,425)
Net Income Including Noncontrolling Interest		684	972	7,000	7,789
Less Net Income Attributable to Noncontrolling Interest, net of tax		0	0	0	0
Net Income (Loss)		684	972	7,000	7,789
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		11,242	11,212	33,568	33,876
Depreciation and amortization		(1,858)	(1,833)	(5,562)	(5,537)
Other operating expenses	[1]	(3,246)	(3,208)	(9,658)	(9,698)
Operating income (loss)	[1]	6,138	6,171	18,348	18,641
Other income (deductions), net	[1]	0	0	0	0
Interest (expense) income, net		(1,371)	(1,436)	(4,170)	(4,348)
Net Income Including Noncontrolling Interest		4,767	4,735	14,178	14,293
Less Net Income Attributable to Noncontrolling Interest, net of tax		0	0	0	0
Net Income (Loss)		4,767	4,735	14,178	14,293
MGE [Member] | Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	[2]	0	0	0	0
Depreciation and amortization	[2]	0	0	0	0
Other operating expenses	[1],[2]	0	4	0	0
Operating income (loss)	[1],[2]	0	4	0	0
Other income (deductions), net	[1],[2]	0	868	0	3,262
Interest (expense) income, net	[2]	0	0	0	0
Net Income Including Noncontrolling Interest	[2]	0	872	0	3,262
Less Net Income Attributable to Noncontrolling Interest, net of tax	[2]	0	0	0	0
Net Income (Loss)	[2]	0	872	0	3,262
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues		(14,912)	(17,486)	(44,669)	(51,614)
Depreciation and amortization		0	0	0	0
Other operating expenses	[1]	14,912	17,486	44,669	51,614
Operating income (loss)	[1]	0	0	0	0
Other income (deductions), net	[1]	0	0	0	0
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		(5,439)	(5,695)	(16,224)	(17,899)
Net Income (Loss)		(5,439)	(5,695)	(16,224)	(17,899)
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		(109)	454	(337)	1,464
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		3,891	6,034	11,699	17,256
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		11,130	10,998	33,307	32,894
MGE [Member] | Consolidation Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	[2]	$ 0	$ 0	$ 0	$ 0

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

[2] As of July 31, 2016, MGE no longer consolidates MGE Energy's proportionate share of equity earnings in MGE Transco. See Footnote 3 for further discussion.