Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2019	Jun. 30, 2018		Dec. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 2,297	$ 1,990		$ 4,515	$ 4,441
Dividends from ATC		1,733	2,959		3,745	3,905
Capital contributions to investments		1,065	1,065		3,386	3,280
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		2,297	1,990		4,515	4,441
Dividends from ATC		1,733	2,959	[1]	3,745	2,959	[1]
Capital contributions to investments		$ 1,065	1,065		$ 1,243	1,775
Dividend receivable from ATC			$ 1,400			$ 1,400		$ 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 proceeding year. MGE Transco recorded a $1.4 million dividend receivable from ATC as of June 30, 2018. A cash dividend was received in July 2018.