Investments (Details-2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments	$ 10,221	$ 9,889	$ 8,821
Dividends from investment	8,998	7,347	6,958
Capital contributions to investments	5,601	7,813	5,926
MGE Energy [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments	92,922	87,728	85,220
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments	10,167	9,889	8,821
Dividends from investment	8,633	7,347	4,611	[1]
Capital contributions to investments	$ 1,249	$ 3,018	2,841
Ownership interest in equity-method investee	3.60%	3.60%
Dividend receivable from ATC					$ 2,300
MGEE Transco [Member] | ATC Holdco [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments	$ 300	$ 100	$ 300
Ownership interest in equity-method investee	4.40%	4.40%

[1] MGE Transco recorded a $2.3 million dividend receivable from ATC as of December 31, 2017. A cash dividend was received in January of 2018.