Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended
Jul. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 3,139	$ 2,297	$ 5,405	$ 4,515
Dividends from ATC		1,880	1,733	4,581	3,745
Capital contributions to investments		355	1,065	2,577	3,386
MGE Transco [Member] \| ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		3,139	2,297	5,405	4,515
Dividends from ATC		1,880	1,733	4,271	3,745
Capital contributions to investments		$ 355	$ 1,065	$ 533	$ 1,243
MGE Transco [Member] \| ATC [Member] \| Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments	$ 400

Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

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