Noncontrolling Interest (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act	[1]		$ 21,651
MGE Power [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent		100.00%
MGE [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet		$ 141,454	140,902
Net income attributable to noncontrolling interest, net of tax		22,552	43,237	$ 23,358
MGE [Member] | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act	[1]		21,649
MGE Transco [Member]
Noncontrolling Interest [Line Items]
Net income attributable to noncontrolling interest, net of tax	[2]	0	0	1,410
MGE Power West Campus [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet	[3]	43,935	43,267
Net income attributable to noncontrolling interest, net of tax	[3]	7,168	13,963	7,200
MGE Power West Campus [Member] | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act			6,800
MGE Power Elm Road [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet	[3]	97,519	97,635
Net income attributable to noncontrolling interest, net of tax	[3]	$ 15,384	29,274	$ 14,748
MGE Power Elm Road [Member] | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act			$ 14,800

[1] Generated by nonregulated activities.

[2] A s of December 31, 2018 , MGE Energy is the owner of MGE Transco. In July 2016, MGE's ownership interest in MGE Transco declined below a majority as a result of continued funding of ATC capital contributions by MGE Energy. As a result of the change in majority owner ship in MGE Transco , MGE deconsolidated MGE Energy's proportionate share of the equity in MGE Transco. The change in co nsolidation was applied prospectively by reducing its investment and noncontrolling interest on MGE's consolidated financial statements. The change had no effect on MGE Energy's consolidated financial statements; however, MGE Energy's proportionate share o f the equity and net income of MGE Transco classified as noncontrolling interest was deconsolidated from MGE ' s financial statements. No gain or loss was recognized in July 2016 due to MGE ceasing to have a controlling financial interest .

[3] MGE Power Elm Road and MGE Power West Campus are not su bsidiaries of MGE; however, they have been consolidated in the consolidated financial s tatements of MGE (see Footnote 3 ). MGE Power Elm Road and MGE Power West Campus are 100 % owned by MGE Power, and MGE Power is 100 % o wned by MGE Energy. MGE Energy's proportionate share of the equity and net income (through its wholly owned subsidiary MGE Power) of MGE Power Elm Road and MGE Power West Campus is classified within the MGE consolidated financial statements as noncontrolling interest . In 2017, there was a $14.8 million and $6.8 million one-time tax impact as a result of the Tax Act for MGE Power Elm Road and MGE Power West Campus, respectively. The Tax Act reduced the f ederal tax rate from 35% to 21%. See Footnote 12 for further information .