Investments (Details-2) - USD ($) $ in Thousands			12 Months Ended
	Jan. 31, 2018	Dec. 01, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments			$ 10,125		$ 8,428		$ 7,728
Dividends from ATC			8,803		5,854		6,645
Capital contributions to investments			11,081		2,958		1,053
MGE Energy [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments			99,246		75,581		71,306
Dividend in kind to parent		$ (15,800)
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments			10,125		8,670		7,728
Dividends from ATC			9,078	[1]	7,926	[1]	6,645
Capital contributions to investments			$ 3,551		$ 2,486		710
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	$ 700
MGEE Transco [Member] | ATC Holdco [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments			$ 2,900
Ownership interest in equity-method investee			4.40%		4.00%
MGEE Transco [Member] | ATC Holdco [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Capital contributions to investments	$ 100
MGE [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings of investments			$ 0		$ 6,366		7,728
Dividends from ATC			0		5,032		6,645
Capital contributions to investments			0		1,598		710
Dividend in kind to parent		$ 15,800	$ 0		$ 15,822		$ 0

[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.