MGE Energy Inc Consolidated Statements of Common Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss)/ Income [Member]
Increase (Decrease) in Equity [Roll Forward]
Cumulative effect of new accounting principle	$ (142)			$ (142)
Beginning balance, shares at Dec. 31, 2014		34,668
Beginning balance, value at Dec. 31, 2014	659,401	$ 34,668	$ 316,268	308,007	$ 458
Beginning balance, value - Adjusted at Dec. 31, 2014	659,259			307,865
Increase (Decrease) in Equity [Roll Forward]
Net Income	71,343			71,343
Other comprehensive income (loss)	(101)				(101)
Common stock dividends declared	(40,043)			(40,043)
Ending balance, shares at Dec. 31, 2015		34,668
Ending balance, value at Dec. 31, 2015	690,458	$ 34,668	316,268	339,165	357
Increase (Decrease) in Equity [Roll Forward]
Net Income	75,560			75,560
Other comprehensive income (loss)	(155)				(155)
Common stock dividends declared	(41,775)			(41,775)
Ending balance, shares at Dec. 31, 2016		34,668
Ending balance, value at Dec. 31, 2016	724,088	$ 34,668	316,268	372,950	202
Increase (Decrease) in Equity [Roll Forward]
Net Income	97,606			97,606
Other comprehensive income (loss)	175				175
Common stock dividends declared	(43,682)			(43,682)
Ending balance, shares at Dec. 31, 2017		34,668
Ending balance, value at Dec. 31, 2017	$ 778,187	$ 34,668	$ 316,268	$ 426,874	$ 377