Segment Information (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 122,147,000	$ 124,262,000	$ 289,716,000	$ 281,894,000
Depreciation and amortization	(18,091,000)	(13,872,000)	(35,230,000)	(27,495,000)
Other operating expenses	(84,273,000)	(86,159,000)	(204,193,000)	(203,980,000)
Operating income (loss)	19,783,000	24,231,000	50,293,000	50,419,000
Other income (deductions), net	5,019,000	4,731,000	9,870,000	9,650,000
Interest (expense) income, net	(5,749,000)	(4,783,000)	(11,396,000)	(9,522,000)
Income before income taxes	19,053,000	24,179,000	48,767,000	50,547,000
Income tax (provision) benefit	(3,505,000)	(5,828,000)	(9,213,000)	(12,195,000)
Net Income	15,548,000	18,351,000	39,554,000	38,352,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	2,026,086,000		2,026,086,000		$ 1,988,618,000
Capital expenditures			79,092,000	100,389,000	212,197,000
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	96,863,000	99,067,000	194,279,000	193,458,000
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	25,070,000	24,980,000	95,170,000	87,745,000
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	214,000	215,000	267,000	691,000
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	122,147,000	124,262,000	289,716,000	281,894,000
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	97,061,000	98,822,000	194,676,000	193,244,000
Depreciation and amortization	(13,379,000)	(9,575,000)	(25,888,000)	(18,953,000)
Other operating expenses	(73,130,000)	(74,535,000)	(146,424,000)	(151,110,000)
Operating income (loss)	10,552,000	14,712,000	22,364,000	23,181,000
Other income (deductions), net	1,788,000	1,562,000	3,441,000	3,292,000
Interest (expense) income, net	(3,826,000)	(2,952,000)	(7,491,000)	(5,819,000)
Income before income taxes	8,514,000	13,322,000	18,314,000	20,654,000
Income tax (provision) benefit	(638,000)	(2,986,000)	(1,088,000)	(4,186,000)
Net Income	7,876,000	10,336,000	17,226,000	16,468,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,250,755,000		1,250,755,000		1,193,083,000
Capital expenditures			62,281,000		176,399,000
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	26,999,000	28,473,000	102,061,000	95,585,000
Depreciation and amortization	(2,843,000)	(2,445,000)	(5,603,000)	(4,840,000)
Other operating expenses	(22,931,000)	(24,232,000)	(84,369,000)	(79,427,000)
Operating income (loss)	1,225,000	1,796,000	12,089,000	11,318,000
Other income (deductions), net	1,170,000	752,000	1,724,000	1,518,000
Interest (expense) income, net	(1,032,000)	(887,000)	(2,072,000)	(1,726,000)
Income before income taxes	1,363,000	1,661,000	11,741,000	11,110,000
Income tax (provision) benefit	(412,000)	(372,000)	(3,031,000)	(2,830,000)
Net Income	951,000	1,289,000	8,710,000	8,280,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	369,467,000		369,467,000		377,005,000
Capital expenditures			15,028,000		30,497,000
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,209,000	10,062,000	20,235,000	20,358,000
Depreciation and amortization	(1,869,000)	(1,852,000)	(3,739,000)	(3,702,000)
Other operating expenses	(33,000)	(34,000)	(81,000)	(77,000)
Operating income (loss)	8,307,000	8,176,000	16,415,000	16,579,000
Other income (deductions), net	0	0	0	0
Interest (expense) income, net	(1,275,000)	(1,334,000)	(2,565,000)	(2,682,000)
Income before income taxes	7,032,000	6,842,000	13,850,000	13,897,000
Income tax (provision) benefit	(1,916,000)	(1,864,000)	(3,773,000)	(3,786,000)
Net Income	5,116,000	4,978,000	10,077,000	10,111,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	261,707,000		261,707,000		265,301,000
Capital expenditures			1,783,000		5,301,000
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	(5,000)	0	(8,000)
Operating income (loss)	0	(5,000)	0	(8,000)
Other income (deductions), net	2,267,000	1,916,000	4,436,000	4,288,000
Interest (expense) income, net	0	0	0	0
Income before income taxes	2,267,000	1,911,000	4,436,000	4,280,000
Income tax (provision) benefit	(619,000)	(522,000)	(1,210,000)	(1,170,000)
Net Income	1,648,000	1,389,000	3,226,000	3,110,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	68,350,000		68,350,000		66,366,000
Capital expenditures			0		0
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Depreciation and amortization	0	0	0	0
Other operating expenses	(301,000)	(448,000)	(575,000)	(651,000)
Operating income (loss)	(301,000)	(448,000)	(575,000)	(651,000)
Other income (deductions), net	(206,000)	501,000	269,000	552,000
Interest (expense) income, net	384,000	390,000	732,000	705,000
Income before income taxes	(123,000)	443,000	426,000	606,000
Income tax (provision) benefit	80,000	(84,000)	(111,000)	(223,000)
Net Income	(43,000)	359,000	315,000	383,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	453,604,000		453,604,000		465,661,000
Capital expenditures			0		0
Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(12,122,000)	(13,095,000)	(27,256,000)	(27,293,000)
Depreciation and amortization	0	0	0	0
Other operating expenses	12,122,000	13,095,000	27,256,000	27,293,000
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	0	0	0	0
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(377,797,000)		(377,797,000)		(378,798,000)
Capital expenditures			0		0
Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	198,000	(245,000)	397,000	(214,000)
Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,929,000	3,493,000	6,891,000	7,840,000
Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	9,995,000	9,847,000	19,968,000	19,667,000
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	122,147,000	124,262,000	289,716,000	281,894,000
Depreciation and amortization	(18,091,000)	(13,872,000)	(35,230,000)	(27,495,000)
Other operating expenses	(83,972,000)	(85,707,000)	(203,618,000)	(203,322,000)
Operating income (loss)	20,084,000	24,683,000	50,868,000	51,077,000
Other income (deductions), net	2,958,000	2,314,000	5,165,000	4,811,000
Interest (expense) income, net	(6,133,000)	(5,173,000)	(12,128,000)	(10,227,000)
Income before income taxes	16,909,000	21,824,000	43,905,000	45,661,000
Income tax (provision) benefit	(2,966,000)	(5,221,000)	(7,892,000)	(10,802,000)
Net Income Including Noncontrolling Interest	13,943,000	16,603,000	36,013,000	34,859,000
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,621,000)	(5,505,000)	(11,111,000)	(11,311,000)
Net Income	8,322,000	11,098,000	24,902,000	23,548,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,881,666,000		1,881,666,000		1,834,891,000
Capital expenditures			79,092,000	100,389,000	212,197,000
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	96,863,000	99,067,000	194,279,000	193,458,000
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	25,070,000	24,980,000	95,170,000	87,745,000
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	214,000	215,000	267,000	691,000
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	122,147,000	124,262,000	289,716,000	281,894,000
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	97,061,000	98,822,000	194,676,000	193,244,000
Depreciation and amortization	(13,379,000)	(9,575,000)	(25,888,000)	(18,953,000)
Other operating expenses	(73,130,000)	(74,535,000)	(146,424,000)	(151,110,000)
Operating income (loss)	10,552,000	14,712,000	22,364,000	23,181,000
Other income (deductions), net	1,788,000	1,561,000	3,441,000	3,292,000
Interest (expense) income, net	(3,826,000)	(2,952,000)	(7,491,000)	(5,819,000)
Income before income taxes	8,514,000	13,321,000	18,314,000	20,654,000
Income tax (provision) benefit	(638,000)	(2,985,000)	(1,088,000)	(4,186,000)
Net Income Including Noncontrolling Interest	7,876,000	10,336,000	17,226,000	16,468,000
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income	7,876,000	10,336,000	17,226,000	16,468,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	1,250,755,000		1,250,755,000		1,193,083,000
Capital expenditures			62,281,000		176,399,000
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	26,999,000	28,473,000	102,061,000	95,585,000
Depreciation and amortization	(2,843,000)	(2,445,000)	(5,603,000)	(4,840,000)
Other operating expenses	(22,931,000)	(24,232,000)	(84,369,000)	(79,427,000)
Operating income (loss)	1,225,000	1,796,000	12,089,000	11,318,000
Other income (deductions), net	1,170,000	753,000	1,724,000	1,519,000
Interest (expense) income, net	(1,032,000)	(887,000)	(2,072,000)	(1,726,000)
Income before income taxes	1,363,000	1,662,000	11,741,000	11,111,000
Income tax (provision) benefit	(412,000)	(373,000)	(3,031,000)	(2,831,000)
Net Income Including Noncontrolling Interest	951,000	1,289,000	8,710,000	8,280,000
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income	951,000	1,289,000	8,710,000	8,280,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	369,467,000		369,467,000		377,005,000
Capital expenditures			15,028,000		30,497,000
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,209,000	10,062,000	20,235,000	20,358,000
Depreciation and amortization	(1,869,000)	(1,852,000)	(3,739,000)	(3,702,000)
Other operating expenses	(33,000)	(35,000)	(81,000)	(78,000)
Operating income (loss)	8,307,000	8,175,000	16,415,000	16,578,000
Other income (deductions), net	0	0	0	0
Interest (expense) income, net	(1,275,000)	(1,334,000)	(2,565,000)	(2,682,000)
Income before income taxes	7,032,000	6,841,000	13,850,000	13,896,000
Income tax (provision) benefit	(1,916,000)	(1,863,000)	(3,773,000)	(3,785,000)
Net Income Including Noncontrolling Interest	5,116,000	4,978,000	10,077,000	10,111,000
Less Net Income Attributable to Noncontrolling Interest, net of tax	0	0	0	0
Net Income	5,116,000	4,978,000	10,077,000	10,111,000
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	261,657,000		261,657,000		265,251,000
Capital expenditures			1,783,000		5,301,000
MGE [Member] | Consolidation Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues	(12,122,000)	(13,095,000)	(27,256,000)	(27,293,000)
Depreciation and amortization	0	0	0	0
Other operating expenses	12,122,000	13,095,000	27,256,000	27,293,000
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 Including Noncontrolling Interest	0	0	0	0
Less Net Income Attributable to Noncontrolling Interest, net of tax	(5,621,000)	(5,505,000)	(11,111,000)	(11,311,000)
Net Income	(5,621,000)	(5,505,000)	(11,111,000)	(11,311,000)
Segment Reporting Assets And Capital Expenditures [Abstract]
Assets	(213,000)		(213,000)		(448,000)
Capital expenditures			0		$ 0
MGE [Member] | Consolidation Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	198,000	(245,000)	397,000	(214,000)
MGE [Member] | Consolidation Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,929,000	3,493,000	6,891,000	7,840,000
MGE [Member] | Consolidation Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 9,995,000	$ 9,847,000	$ 19,968,000	$ 19,667,000