Noncontrolling Interest (Details) - USD ($)		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act				$ 21,700,000
MGE Power [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent		100.00%
MGE [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet		$ 140,303,000	$ 141,454,000
Net income attributable to noncontrolling interest, net of tax		22,349,000	22,552,000	43,237,000
Effects Of The Tax Cuts And Jobs Act [Abstract]
Net Deferred Income Tax Benefit Tax Cuts And Jobs Act				21,700,000
MGE Power West Campus [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet	[1]	43,131,000	43,935,000
Net income attributable to noncontrolling interest, net of tax	[1]	7,196,000	7,168,000	13,963,000
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				6,800,000
MGE Power Elm Road [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet	[1]	97,172,000	97,519,000
Net income attributable to noncontrolling interest, net of tax	[1]	$ 15,153,000	$ 15,384,000	29,274,000
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				$ 14,800,000

[1] MGE Power Elm Road and MGE Power West Campus are not subsidiaries of MGE; however, they have been consolidated in the consolidated financial statements 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% owned 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 federal tax rate from 35% to 21%.