Investments (Details-2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments	$ 8,821		$ 10,125		$ 8,428
Dividends from investment	6,958		8,803		5,854
Capital contributions to investments	5,926		11,081		2,958
MGE Energy [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments	85,220		99,246		75,581
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments	8,821		10,125		8,670
Dividends from investment	4,611	[1]	9,078	[1]	7,926
Capital contributions to investments	$ 2,841		$ 3,551		2,486
Ownership interest in equity-method investee	3.60%		3.60%
Dividend receivable from ATC			$ 2,300		2,100
MGEE Transco [Member] | ATC Holdco [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments	$ 300		$ 2,900
Ownership interest in equity-method investee	4.40%		4.40%
MGE [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments	$ 0		$ 0		6,366
Dividends from investment	0		0		5,032
Capital contributions to investments	0		0		1,598
Dividend in kind to parent	$ 0		$ 0		$ 15,822

[1] MGE Transco recorded a $ 2.3 million and a $ 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.