Noncontrolling Interest (Details) - USD ($)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
MGE Power [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent		100.00%
MGE [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet		$ 141,196,000	$ 140,303,000
Net income attributable to noncontrolling interest, net of tax		22,393,000	22,349,000	$ 22,552,000
MGE Power West Campus [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet	[1]	44,340,000	43,131,000
Net income attributable to noncontrolling interest, net of tax	[1]	7,209,000	7,196,000	7,168,000
MGE Power Elm Road [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet	[1]	96,856,000	97,172,000
Net income attributable to noncontrolling interest, net of tax	[1]	$ 15,184,000	$ 15,153,000	$ 15,384,000

[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.