Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 167,569	$ 157,632
Depreciation and amortization	(17,139)	(13,623)
Other operating expenses	(119,918)	(117,821)
Operating income (loss)	30,512	26,188
Other income (deductions), net	4,851	4,919
Interest (expense) income, net	(5,647)	(4,739)
Income before income taxes	29,716	26,368
Income tax (provision) benefit	(5,709)	(6,367)
Net Income	24,007	20,001
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	2,003,566		$ 1,988,618
Capital expenditures	31,933	37,631	212,197
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	97,416	94,391
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	70,100	62,765
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	53	476
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	167,569	157,632
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	97,615	94,422
Depreciation and amortization	(12,509)	(9,378)
Other operating expenses	(73,294)	(76,575)
Operating income (loss)	11,812	8,469
Other income (deductions), net	1,653	1,730
Interest (expense) income, net	(3,665)	(2,867)
Income before income taxes	9,800	7,332
Income tax (provision) benefit	(451)	(1,200)
Net Income	9,349	6,132
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,223,815		1,193,083
Capital expenditures	24,095		176,399
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	75,062	67,112
Depreciation and amortization	(2,760)	(2,395)
Other operating expenses	(61,438)	(55,195)
Operating income (loss)	10,864	9,522
Other income (deductions), net	554	766
Interest (expense) income, net	(1,040)	(839)
Income before income taxes	10,378	9,449
Income tax (provision) benefit	(2,618)	(2,458)
Net Income	7,760	6,991
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	368,043		377,005
Capital expenditures	7,145		30,497
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,026	10,296
Depreciation and amortization	(1,870)	(1,850)
Other operating expenses	(48)	(43)
Operating income (loss)	8,108	8,403
Other income (deductions), net	0	0
Interest (expense) income, net	(1,290)	(1,348)
Income before income taxes	6,818	7,055
Income tax (provision) benefit	(1,857)	(1,922)
Net Income	4,961	5,133
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	262,979		265,301
Capital expenditures	693		5,301
Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Depreciation and amortization	0	0
Other operating expenses	0	(3)
Operating income (loss)	0	(3)
Other income (deductions), net	2,168	2,372
Interest (expense) income, net	0	0
Income before income taxes	2,168	2,369
Income tax (provision) benefit	(591)	(648)
Net Income	1,577	1,721
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	66,720		66,366
Capital expenditures	0		0
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Depreciation and amortization	0	0
Other operating expenses	(272)	(203)
Operating income (loss)	(272)	(203)
Other income (deductions), net	476	51
Interest (expense) income, net	348	315
Income before income taxes	552	163
Income tax (provision) benefit	(192)	(139)
Net Income	360	24
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	461,159		465,661
Capital expenditures	0		0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(15,134)	(14,198)
Depreciation and amortization	0	0
Other operating expenses	15,134	14,198
Operating income (loss)	0	0
Other income (deductions), net	0	0
Interest (expense) income, net	0	0
Income before income taxes	0	0
Income tax (provision) benefit	0	0
Net Income	0	0
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(379,150)		(378,798)
Capital expenditures	0		0
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	199	31
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,962	4,347
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	9,973	9,820
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	167,569	157,632
Depreciation and amortization	(17,139)	(13,623)
Other operating expenses	(119,646)	117,615
Operating income (loss)	30,784	26,394
Other income (deductions), net	2,207	2,497
Interest (expense) income, net	(5,995)	(5,054)
Income before income taxes	26,996	23,837
Income tax (provision) benefit	(4,926)	(5,581)
Net Income Including Noncontrolling Interest	22,070	18,256
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,490)	(5,806)
Net Income	16,580	12,450
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,853,797		1,834,891
Capital expenditures	31,933	37,631	212,197
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	97,416	94,391
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	70,100	62,765
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	53	476
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	167,569	157,632
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	97,615	94,422
Depreciation and amortization	(12,509)	(9,378)
Other operating expenses	(73,294)	(76,575)
Operating income (loss)	11,812	8,469
Other income (deductions), net	1,653	1,731
Interest (expense) income, net	(3,665)	(2,867)
Income before income taxes	9,800	7,333
Income tax (provision) benefit	(451)	(1,201)
Net Income Including Noncontrolling Interest	9,349	6,132
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0
Net Income	9,349	6,132
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,223,815		1,193,083
Capital expenditures	24,095		176,399
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	75,062	67,112
Depreciation and amortization	(2,760)	(2,395)
Other operating expenses	(61,438)	(55,195)
Operating income (loss)	10,864	9,522
Other income (deductions), net	554	766
Interest (expense) income, net	(1,040)	(839)
Income before income taxes	10,378	9,449
Income tax (provision) benefit	(2,618)	(2,458)
Net Income Including Noncontrolling Interest	7,760	6,991
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0
Net Income	7,760	6,991
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	368,043		377,005
Capital expenditures	7,145		30,497
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,026	10,296
Depreciation and amortization	(1,870)	(1,850)
Other operating expenses	(48)	(43)
Operating income (loss)	8,108	8,403
Other income (deductions), net	0	0
Interest (expense) income, net	(1,290)	(1,348)
Income before income taxes	6,818	7,055
Income tax (provision) benefit	(1,857)	(1,922)
Net Income Including Noncontrolling Interest	4,961	5,133
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0
Net Income	4,961	5,133
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	262,929		265,251
Capital expenditures	693		5,301
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(15,134)	(14,198)
Depreciation and amortization	0	0
Other operating expenses	15,134	14,198
Operating income (loss)	0	0
Other income (deductions), net	0	0
Interest (expense) income, net	0	0
Income before income taxes	0	0
Income tax (provision) benefit	0	0
Net Income Including Noncontrolling Interest	0	0
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,490)	(5,806)
Net Income	(5,490)	(5,806)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(990)		(448)
Capital expenditures	0		$ 0
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	199	31
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	4,962	4,347
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 9,973	$ 9,820