Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands			3 Months Ended		9 Months Ended
		Oct. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC			$ 1,672	$ 2,338	$ 6,113	$ 7,432
Dividends from ATC			1,581	2,070	5,336	6,142
Capital contributions to investments			533	710	4,801	6,863
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC			1,672	2,338	6,113	7,432
Dividends from ATC	[1]		1,581	2,070	4,540	4,070
Capital contributions to investments			$ 533	710	$ 2,308	3,018
Ownership interest in equity-method investee			3.60%		3.60%		3.60%
Dividend receivable from ATC			$ 1,600		$ 1,600		$ 2,300	$ 2,100
MGE Transco [Member] | ATC [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments		$ 500
MGEE Transco [Member] | ATC Holdco [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments			$ 100	$ 0	$ 300	$ 700
Ownership interest in equity-method investee			4.40%		4.40%		4.40%
MGEE Transco [Member] | ATC Holdco [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments		$ 100

[1] MGE Transco recorded a $2.3 million and $2.1 million dividend receivable from ATC as of December 31, 2017 and 2016, respectively. A cash dividend was received in January of each of the proceeding years . MGE Transco recorded a $ 1.6 million dividend receivable from ATC as of September 30, 2018 . A cash dividend was received in October 2018 .