MGE Energy Inc Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Other Comprehensive Income [Abstract]
Net Income	$ 36,134	$ 26,629	$ 15,543	$ 19,300	$ 14,502	$ 27,880	$ 16,150	$ 17,028	$ 97,606	$ 75,560	$ 71,343
Other comprehensive income, net of tax:
Unrealized gain (loss) on available for sale securities, net of tax									175	(155)	(101)
Comprehensive Income									$ 97,781	$ 75,405	$ 71,242