Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues		$ 157,632	$ 156,823
Depreciation and amortization		(13,623)	(12,959)
Other operating expenses		(117,821)	(112,028)	[1]
Operating income (loss)		26,188	31,836
Other income (deductions), net		4,919	3,525	[1]
Interest (expense) income, net		(4,739)	(4,894)
Income before income taxes		26,368	30,467
Income tax (provision) benefit		(6,367)	(11,167)
Net Income (Loss)		20,001	19,300
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		94,391	98,324
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		62,765	58,426
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		476	73
Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues		0	0
All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues		0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues		157,632	156,823
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		94,422	98,219
Depreciation and amortization		(9,378)	(8,900)
Other operating expenses		(76,575)	(76,006)	[1]
Operating income (loss)		8,469	13,313
Other income (deductions), net		1,730	870	[1]
Interest (expense) income, net		(2,867)	(2,776)
Income before income taxes		7,332	11,407
Income tax (provision) benefit		(1,200)	(3,454)
Net Income (Loss)		6,132	7,953
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		67,112	62,820
Depreciation and amortization		(2,395)	(2,213)
Other operating expenses		(55,195)	(50,971)	[1]
Operating income (loss)		9,522	9,636
Other income (deductions), net		766	399	[1]
Interest (expense) income, net		(839)	(799)
Income before income taxes		9,449	9,236
Income tax (provision) benefit		(2,458)	(3,704)
Net Income (Loss)		6,991	5,532
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		10,296	11,143
Depreciation and amortization		(1,850)	(1,846)
Other operating expenses		(43)	(41)	[1]
Operating income (loss)		8,403	9,256
Other income (deductions), net		0	0	[1]
Interest (expense) income, net		(1,348)	(1,407)
Income before income taxes		7,055	7,849
Income tax (provision) benefit		(1,922)	(3,151)
Net Income (Loss)		5,133	4,698
Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues		0	0
Depreciation and amortization		0	0
Other operating expenses		(3)	0	[1]
Operating income (loss)		(3)	0
Other income (deductions), net		2,372	2,478	[1]
Interest (expense) income, net		0	0
Income before income taxes		2,369	2,478
Income tax (provision) benefit		(648)	(997)
Net Income (Loss)		1,721	1,481
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues		0	0
Depreciation and amortization		0	0
Other operating expenses		(203)	(369)	[1]
Operating income (loss)		(203)	(369)
Other income (deductions), net		51	(222)	[1]
Interest (expense) income, net		315	88
Income before income taxes		163	(503)
Income tax (provision) benefit		(139)	139
Net Income (Loss)		24	(364)
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues		(14,198)	(15,359)
Depreciation and amortization		0	0
Other operating expenses		14,198	15,359	[1]
Operating income (loss)		0	0
Other income (deductions), net		0	0	[1]
Interest (expense) income, net		0	0
Income before income taxes		0	0
Income tax (provision) benefit		0	0
Net Income (Loss)		0	0
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		31	(105)
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		4,347	4,394
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		9,820	11,070
Consolidation Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues		0	0
Consolidation Elimination Entries [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues		0	0
MGE [Member]
Segment Reporting Information [Line Items]
Operating revenues		157,632	156,836
Depreciation and amortization		(13,623)	(12,959)
Other operating expenses	[2]	(123,139)	(121,937)	[1]
Operating income (loss)	[2]	20,870	21,940
Other income (deductions), net	[2]	2,440	1,225	[1]
Interest (expense) income, net		(5,054)	(4,982)
Net Income Including Noncontrolling Interest		18,256	18,183
Less Net Income Attributable to Noncontrolling Interest, net of tax		(5,806)	(5,389)
Net Income (Loss)		12,450	12,794
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		94,391	98,328
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		62,765	58,435
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		476	73
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues		157,632	156,836
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		94,422	98,219
Depreciation and amortization		(9,378)	(8,900)
Other operating expenses	[2]	(77,731)	(79,426)	[1]
Operating income (loss)	[2]	7,313	9,893
Other income (deductions), net	[2]	1,686	836	[1]
Interest (expense) income, net		(2,867)	(2,776)
Net Income Including Noncontrolling Interest		6,132	7,953
Less Net Income Attributable to Noncontrolling Interest, net of tax		0	0
Net Income (Loss)		6,132	7,953
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		67,112	62,820
Depreciation and amortization		(2,395)	(2,213)
Other operating expenses	[2]	(57,641)	(54,665)	[1]
Operating income (loss)	[2]	7,076	5,942
Other income (deductions), net	[2]	754	389	[1]
Interest (expense) income, net		(839)	(799)
Net Income Including Noncontrolling Interest		6,991	5,532
Less Net Income Attributable to Noncontrolling Interest, net of tax		0	0
Net Income (Loss)		6,991	5,532
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		10,296	11,143
Depreciation and amortization		(1,850)	(1,846)
Other operating expenses	[2]	(1,965)	(3,192)	[1]
Operating income (loss)	[2]	6,481	6,105
Other income (deductions), net	[2]	0	0	[1]
Interest (expense) income, net		(1,348)	(1,407)
Net Income Including Noncontrolling Interest		5,133	4,698
Less Net Income Attributable to Noncontrolling Interest, net of tax		0	0
Net Income (Loss)		5,133	4,698
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues		(14,198)	(15,346)
Depreciation and amortization		0	0
Other operating expenses	[2]	14,198	15,346	[1]
Operating income (loss)	[2]	0	0
Other income (deductions), net	[2]	0	0	[1]
Interest (expense) income, net		0	0
Net Income Including Noncontrolling Interest		0	0
Less Net Income Attributable to Noncontrolling Interest, net of tax		(5,806)	(5,389)
Net Income (Loss)		(5,806)	(5,389)
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues		31	(109)
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues		4,347	4,385
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues		$ 9,820	$ 11,070

[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.