Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Dec. 01, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 2,338	$ 2,023	$ 7,432		$ 6,023
Dividends from ATC		2,070	1,486	6,142		4,214
Capital contributions to investments		710	888	6,863		2,036
MGE Energy [Member]
Schedule Of Equity Method Investments [Line Items]
Dividend in kind to parent	$ 15,800
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		2,338	2,023	7,432		6,023
Dividends from ATC		2,070	1,486	4,070	[1]	4,214
Capital contributions to investments		$ 710	888	$ 3,018		1,598
Ownership interest in equity-method investee		3.60%		3.60%			3.60%
Dividend receivable from ATC							$ 2,100
MGE [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 0	$ 1,451	$ 0		5,451
Dividends from ATC				0		4,214
Capital contributions to investments				$ 0		$ 1,598
Dividend in kind to parent	$ 15,800
MGEE Transco [Member] | ATC Holdco [Member]
Schedule Of Equity Method Investments [Line Items]
Ownership interest in equity-method investee		4.40%		4.40%			4.00%

[1] As of December 31, 2016, MGE Transco recorded a $2.1 million receivable from ATC for a cash dividend received in January 2017.