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Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment				$ 7,780	$ 6,879
Dividends from investments				6,929	5,526
Capital contributions to investments				4,007	5,894
MGE Transco [Member] \| ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment		$ 2,319	$ 2,364	7,724	6,879
Dividends from investments		2,349	1,781	6,620	5,526
Capital contributions to investments		$ 359	$ 888	$ 892	$ 2,131
MGE Transco [Member] \| ATC [Member] \| Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments	$ 400