Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands			3 Months Ended		6 Months Ended
		Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC			$ 1,990	$ 2,486	$ 4,441	$ 5,094
Dividends from ATC			2,959	2,000	3,905	4,072
Capital contributions to investments			1,065	888	3,280	5,557
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC			1,990	2,486	4,441	5,094
Dividends from ATC	[1]		2,959	2,000	2,959	2,000
Capital contributions to investments			$ 1,065	888	$ 1,775	2,308
Ownership interest in equity-method investee			3.60%		3.60%		3.60%
Dividend receivable from ATC			$ 1,400		$ 1,400		$ 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	$ 500	$ 200	$ 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 $ 1.4 million dividend receivable from ATC as of June 30, 2018 . A cash dividend was received in July 2018 .