Investment in ATC and ATC Holdco (Details-1) - USD ($) $ in Thousands		3 Months Ended
	Dec. 01, 2016	Mar. 31, 2017		Mar. 31, 2016	Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		$ 2,608		$ 2,233
Dividends from ATC		2,072		865
Capital contributions to ATC		2,939		671
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity earnings from investment in ATC		2,608		2,233
Dividends from ATC		0	[1]	865
Capital contributions to ATC		$ 1,420		533
Ownership interest in equity-method investee		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		2,233
Dividends from ATC		0		865
Capital contributions to ATC		$ 0		$ 533
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		4.00%			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.