Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
	Oct. 31, 2019	Sep. 30, 2019	Sep. 30, 2018		Sep. 30, 2019	Sep. 30, 2018		Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 2,364	$ 1,672		$ 6,879	$ 6,113
Dividends from ATC		1,781	1,581		5,526	5,336
Capital contributions to investments		888	533		5,894	4,801
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		2,364	1,672		6,879	6,113
Dividends from ATC		1,781	1,581	[1]	5,526	4,540	[1]
Capital contributions to investments		$ 888	533		$ 2,131	2,308
Dividend receivable from ATC			$ 1,600			$ 1,600		$ 2,300
MGE Transco [Member] | ATC [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments	$ 900

[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 following year. MGE Transco recorded a $1.6 million dividend receivable from ATC as of September 30, 2018. A cash dividend was received in October 2018.