Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands		3 Months Ended
	Apr. 30, 2019	Mar. 31, 2019	Mar. 31, 2018		Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 2,218	$ 2,451
Dividends from ATC		2,012	2,348
Capital contributions to investments		740	1,877
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		2,218	2,451
Dividends from ATC		2,012	0	[1]
Capital contributions to investments		$ 178	$ 710
Dividend receivable from ATC					$ 2,300
MGE Transco [Member] | ATC [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments	$ 1,100

[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 the proceeding year.