Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Dec. 01, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 2,486	$ 1,767	$ 5,094		$ 4,000
Dividends from ATC		2,000	1,863	4,072		2,728
Capital contributions to ATC		888	177	5,557		998
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		2,486	1,767	5,094		4,000
Dividends from ATC		2,000	1,863	2,000	[1]	2,728
Capital contributions to ATC		$ 888	177	$ 2,308		710
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	$ 1,767	$ 0		4,000
Dividends from ATC				0		2,728
Capital contributions to ATC				$ 0		$ 710
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		4.30%		4.30%			4.00%
MGE Energy [Member]
Schedule Of Equity Method Investments [Line Items]
Dividend in kind to parent	$ (15,800)

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