Common Equity (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2020 $ / shares	Sep. 30, 2020 $ / shares	Jun. 30, 2020 $ / shares	Mar. 31, 2020 $ / shares	Dec. 31, 2019 $ / shares	Sep. 30, 2019 $ / shares	Jun. 30, 2019 $ / shares	Mar. 31, 2019 $ / shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares
Class of Stock [Line Items]
Common stock dividends declared									$ 51,729,000	$ 47,842,000	$ 45,762,000
Common stock dividends declared (in dollars per share) | $ / shares	$ 0.370	$ 0.370	$ 0.353	$ 0.353	$ 0.353	$ 0.353	$ 0.338	$ 0.338	$ 1.45	$ 1.38	$ 1.32
Common stock issued during period | shares									1,500,000
Common stock issued during period under the stock plan | shares									0	0
Proceeds From Issuance Of Common Stock									$ 79,635,000	$ 0	$ 0
Dilutive Securities									0
MGE Energy [Member]
Class of Stock [Line Items]
Proceeds From Issuance Of Common Stock									79,635,000	0	$ 0
MGE [Member]
Class of Stock [Line Items]
Cash dividends paid to parent by MGE									$ 0	$ 0
MGE [Member] | Minimum [Member]
Class of Stock [Line Items]
Dividend restrictions, common equity ratio									0.55