Segment Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 140,941,000	$ 138,198,000	$ 122,147,000	$ 167,569,000	$ 140,079,000	$ 137,795,000	$ 124,262,000	$ 157,632,000	$ 568,855,000	$ 559,768,000	$ 563,099,000
Depreciation and amortization									(71,562,000)	(56,412,000)	(53,077,000)
Other operating expenses									(386,383,000)	(389,149,000)	(385,397,000)
Operating Income (Loss)	21,877,000	38,738,000	19,783,000	30,512,000	23,990,000	39,798,000	24,231,000	26,188,000	110,910,000	114,207,000	124,625,000
Other (deductions) income, net									18,811,000	17,055,000	14,399,000
Interest (expense) income, net									(23,063,000)	(19,609,000)	(19,324,000)
Income before income taxes									106,658,000	111,653,000	119,700,000
Income tax (provision) benefit	(3,116,000)	(7,454,000)	(3,505,000)	(5,709,000)	(5,642,000)	(9,597,000)	(5,828,000)	(6,367,000)	(19,784,000)	(27,434,000)	(22,094,000)	[1]
Net Income Including Noncontrolling Interest									86,874,000	84,219,000	97,606,000
Net Income	16,662,000	$ 30,657,000	$ 15,548,000	$ 24,007,000	16,361,000	$ 29,506,000	$ 18,351,000	$ 20,001,000	86,874,000	84,219,000	97,606,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	2,081,664,000				1,988,618,000				2,081,664,000	1,988,618,000	1,855,182,000	[2]
Capital expenditures									164,036,000	212,197,000	108,131,000
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act											21,700,000
Investment in ATC	71,648,000				66,339,000				71,648,000	66,339,000	20,400,000
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									408,285,000	400,882,000	413,926,000
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									159,875,000	157,767,000	148,825,000
Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									695,000	1,119,000	348,000
Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									568,855,000	559,768,000	563,099,000
Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									409,091,000	400,593,000	413,416,000
Depreciation and amortization									(52,755,000)	(38,925,000)	(36,660,000)
Other operating expenses									(297,156,000)	(297,374,000)	(308,100,000)
Operating Income (Loss)									59,180,000	64,294,000	68,656,000
Other (deductions) income, net									7,153,000	7,294,000	4,539,000
Interest (expense) income, net									(14,978,000)	(12,198,000)	(11,257,000)
Income before income taxes									51,355,000	59,390,000	61,938,000
Income tax (provision) benefit									(5,037,000)	(13,453,000)	(20,547,000)
Net Income									46,318,000	45,937,000	41,391,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	1,308,277,000				1,193,083,000				1,308,277,000	1,193,083,000	1,058,988,000
Capital expenditures									125,086,000	176,399,000	77,353,000
Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									175,945,000	173,843,000	163,799,000
Depreciation and amortization									(11,320,000)	(10,060,000)	(9,000,000)
Other operating expenses									(145,097,000)	(145,883,000)	(135,028,000)
Operating Income (Loss)									19,528,000	17,900,000	19,771,000
Other (deductions) income, net									3,550,000	3,132,000	1,911,000
Interest (expense) income, net									(4,237,000)	(3,692,000)	(3,234,000)
Income before income taxes									18,841,000	17,340,000	18,448,000
Income tax (provision) benefit									(4,753,000)	(4,474,000)	(7,303,000)
Net Income									14,088,000	12,866,000	11,145,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	408,001,000				377,005,000				408,001,000	377,005,000	354,875,000
Capital expenditures									36,193,000	30,497,000	26,847,000
Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									40,715,000	40,645,000	44,816,000
Depreciation and amortization									(7,487,000)	(7,427,000)	(7,417,000)
Other operating expenses									(144,000)	(144,000)	(190,000)
Operating Income (Loss)									33,084,000	33,074,000	37,209,000
Other (deductions) income, net									0	0	0
Interest (expense) income, net									(5,083,000)	(5,307,000)	(5,533,000)
Income before income taxes									28,001,000	27,767,000	31,676,000
Income tax (provision) benefit									(7,628,000)	(7,534,000)	8,860,000	[1]
Net Income									20,373,000	20,233,000	40,536,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	258,004,000				265,301,000				258,004,000	265,301,000	270,384,000
Capital expenditures									2,757,000	5,301,000	3,931,000
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act											21,700,000
Operating Segments [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
Depreciation and amortization									0	0	0
Other operating expenses									0	(16,000)	(9,000)
Operating Income (Loss)									0	(16,000)	(9,000)
Other (deductions) income, net									9,547,000	8,602,000	9,844,000
Interest (expense) income, net									0	0	0
Income before income taxes									9,547,000	8,586,000	9,835,000
Income tax (provision) benefit									(2,602,000)	(2,345,000)	(3,954,000)
Net Income									6,945,000	6,241,000	5,881,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	71,668,000				66,366,000				71,668,000	66,366,000	61,783,000	[2]
Capital expenditures									0	0	0
Effects Of The Tax Cuts And Jobs Act [Abstract]
Investment in ATC											20,400,000
Operating Segments [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
Depreciation and amortization									0	0	0
Other operating expenses									(882,000)	(1,045,000)	(1,002,000)
Operating Income (Loss)									(882,000)	(1,045,000)	(1,002,000)
Other (deductions) income, net									(1,439,000)	(1,973,000)	(1,895,000)
Interest (expense) income, net									1,235,000	1,588,000	700,000
Income before income taxes									(1,086,000)	(1,430,000)	(2,197,000)
Income tax (provision) benefit									236,000	372,000	850,000
Net Income									(850,000)	(1,058,000)	(1,347,000)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	443,278,000				465,661,000				443,278,000	465,661,000	485,548,000
Capital expenditures									0	0	0
Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues									(56,896,000)	(55,313,000)	(58,932,000)
Depreciation and amortization									0	0	0
Other operating expenses									56,896,000	55,313,000	58,932,000
Operating Income (Loss)									0	0	0
Other (deductions) income, net									0	0	0
Interest (expense) income, net									0	0	0
Income before income taxes									0	0	0
Income tax (provision) benefit									0	0	0
Net Income									0	0	0
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	(407,564,000)				(378,798,000)				(407,564,000)	(378,798,000)	(376,396,000)
Capital expenditures									0	0	0
Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									806,000	(289,000)	(510,000)
Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									16,070,000	16,076,000	14,974,000
Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									40,020,000	39,526,000	44,468,000
Consolidation/Elimination Entries [Member] | Transmission Investment [Member]
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
Consolidation/Elimination Entries [Member] | All Others [Member]
Segment Reporting Information [Line Items]
Operating revenues									0	0	0
MGE [Member]
Segment Reporting Information [Line Items]
Operating revenues									568,855,000	559,768,000	563,111,000
Depreciation and amortization									(71,562,000)	(56,412,000)	(53,077,000)
Other operating expenses									(385,501,000)	(388,088,000)	(384,398,000)
Operating Income (Loss)									111,792,000	115,268,000	125,636,000
Other (deductions) income, net									10,703,000	10,426,000	6,450,000
Interest (expense) income, net									(24,298,000)	(21,197,000)	(20,024,000)
Income before income taxes									98,197,000	104,497,000	112,062,000
Income tax (provision) benefit									(17,418,000)	(25,461,000)	(18,990,000)	[3]
Net Income Including Noncontrolling Interest									80,779,000	79,036,000	93,072,000
Less Net Income Attributable to Noncontrolling Interest, net of tax									(22,349,000)	(22,552,000)	(43,237,000)
Net Income									58,430,000	56,484,000	49,835,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	1,973,477,000				1,834,891,000				1,973,477,000	1,834,891,000	1,684,041,000
Capital expenditures									164,036,000	212,197,000	108,131,000
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act											21,700,000
Investment in ATC	0				0				0	0
MGE [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									408,285,000	400,882,000	413,929,000
MGE [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									159,875,000	157,767,000	148,834,000
MGE [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									695,000	1,119,000	348,000
MGE [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues									568,855,000	559,768,000	563,111,000
MGE [Member] | Operating Segments [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									409,091,000	400,593,000	413,416,000
Depreciation and amortization									(52,755,000)	(38,925,000)	(36,660,000)
Other operating expenses									(297,156,000)	(297,374,000)	(308,100,000)
Operating Income (Loss)									59,180,000	64,294,000	68,656,000
Other (deductions) income, net									7,153,000	7,294,000	4,539,000
Interest (expense) income, net									(14,978,000)	(12,198,000)	(11,257,000)
Income before income taxes									51,355,000	59,390,000	61,938,000
Income tax (provision) benefit									(5,037,000)	(13,453,000)	(20,547,000)
Net Income Including Noncontrolling Interest									46,318,000	45,937,000	41,391,000
Less Net Income Attributable to Noncontrolling Interest, net of tax									0	0	0
Net Income									46,318,000	45,937,000	41,391,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	1,308,277,000				1,193,083,000				1,308,277,000	1,193,083,000	1,058,988,000
Capital expenditures									125,086,000	176,399,000	77,353,000
MGE [Member] | Operating Segments [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									175,945,000	173,843,000	163,799,000
Depreciation and amortization									(11,320,000)	(10,060,000)	(9,000,000)
Other operating expenses									(145,097,000)	(145,883,000)	(135,028,000)
Operating Income (Loss)									19,528,000	17,900,000	19,771,000
Other (deductions) income, net									3,550,000	3,132,000	1,911,000
Interest (expense) income, net									(4,237,000)	(3,692,000)	(3,234,000)
Income before income taxes									18,841,000	17,340,000	18,448,000
Income tax (provision) benefit									(4,753,000)	(4,474,000)	(7,303,000)
Net Income Including Noncontrolling Interest									14,088,000	12,866,000	11,145,000
Less Net Income Attributable to Noncontrolling Interest, net of tax									0	0	0
Net Income									14,088,000	12,866,000	11,145,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	408,001,000				377,005,000				408,001,000	377,005,000	354,875,000
Capital expenditures									36,193,000	30,497,000	26,847,000
MGE [Member] | Operating Segments [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									40,715,000	40,645,000	44,816,000
Depreciation and amortization									(7,487,000)	(7,427,000)	(7,417,000)
Other operating expenses									(144,000)	(144,000)	(190,000)
Operating Income (Loss)									33,084,000	33,074,000	37,209,000
Other (deductions) income, net									0	0	0
Interest (expense) income, net									(5,083,000)	(5,307,000)	(5,533,000)
Income before income taxes									28,001,000	27,767,000	31,676,000
Income tax (provision) benefit									(7,628,000)	(7,534,000)	8,860,000	[3]
Net Income Including Noncontrolling Interest									20,373,000	20,233,000	40,536,000
Less Net Income Attributable to Noncontrolling Interest, net of tax									0	0	0
Net Income									20,373,000	20,233,000	40,536,000
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	257,954,000				265,251,000				257,954,000	265,251,000	270,334,000
Capital expenditures									2,757,000	5,301,000	3,931,000
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act											21,700,000
MGE [Member] | Consolidation/Elimination Entries [Member]
Segment Reporting Information [Line Items]
Operating revenues									(56,896,000)	(55,313,000)	(58,920,000)
Depreciation and amortization									0	0	0
Other operating expenses									56,896,000	55,313,000	58,920,000
Operating Income (Loss)									0	0	0
Other (deductions) income, net									0	0	0
Interest (expense) income, net									0	0	0
Income before income taxes									0	0	0
Income tax (provision) benefit									0	0	0
Net Income Including Noncontrolling Interest									0	0	0
Less Net Income Attributable to Noncontrolling Interest, net of tax									(22,349,000)	(22,552,000)	(43,237,000)
Net Income									(22,349,000)	(22,552,000)	(43,237,000)
Segment Reporting Information Assets And Capital Expenditures [Abstract]
Assets	$ (755,000)				$ (448,000)				(755,000)	(448,000)	(156,000)
Capital expenditures									0	0	0
MGE [Member] | Consolidation/Elimination Entries [Member] | Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues									806,000	(289,000)	(513,000)
MGE [Member] | Consolidation/Elimination Entries [Member] | Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues									16,070,000	16,076,000	14,965,000
MGE [Member] | Consolidation/Elimination Entries [Member] | Non Regulated Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues									$ 40,020,000	$ 39,526,000	$ 44,468,000

[1] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act.

[2] In December 2017, there was a $20.4 million one-time tax impact as a result of the Tax Act.

[3] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act.