Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands			3 Months Ended
		Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC			$ 2,451	$ 2,608
Dividends from ATC			2,348	2,072
Capital contributions to investments			1,877	2,939
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC			2,451	2,608
Dividends from ATC	[1]		0	0
Capital contributions to investments			$ 710	1,420
Ownership interest in equity-method investee			3.60%		3.60%
Dividend receivable from ATC			$ 2,300		$ 2,100
MGE Transco [Member] | ATC [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments		$ 1,100
MGEE Transco [Member] | ATC Holdco [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments			$ 100	$ 200
Ownership interest in equity-method investee			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 .