Common Equity (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares
Class of Stock [Line Items]
Common stock dividends declared	$ 54,788,000	$ 51,729,000	$ 47,842,000
Common stock dividends declared (in dollars per share) | $ / shares	$ 1.52	$ 1.45	$ 1.38
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	$ 0	$ 79,635,000	$ 0
Dilutive Securities	3,668,000
MGE [Member]
Class of Stock [Line Items]
Cash dividends paid to parent by MGE	$ 5,000,000.0	$ 0
MGE [Member] | Minimum [Member]
Class of Stock [Line Items]
Dividend restrictions, common equity ratio	0.55