MGE Energy Inc Consolidated Statements of Common Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income/(Loss) [Member]
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle	$ 0			$ 377	$ (377)
Beginning balance, shares at Dec. 31, 2017		34,668
Beginning balance, value at Dec. 31, 2017	778,187	$ 34,668	$ 316,268	426,874	377
Beginning balance, value - Adjusted at Dec. 31, 2017	778,187			427,251	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	38,352			38,352
Common stock dividends declared	(22,361)			(22,361)
Ending balance, shares at Jun. 30, 2018		34,668
Ending balance, value at Jun. 30, 2018	794,178	$ 34,668	316,268	443,242	0
Beginning balance, shares at Mar. 31, 2018		34,668
Beginning balance, value at Mar. 31, 2018	787,007	$ 34,668	316,268	436,071	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	18,351			18,351
Common stock dividends declared	(11,180)			(11,180)
Ending balance, shares at Jun. 30, 2018		34,668
Ending balance, value at Jun. 30, 2018	794,178	$ 34,668	316,268	443,242	0
Beginning balance, shares at Dec. 31, 2018		34,668
Beginning balance, value at Dec. 31, 2018	816,644	$ 34,668	316,268	465,708	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	39,554			39,554
Common stock dividends declared	(23,401)			(23,401)
Ending balance, shares at Jun. 30, 2019		34,668
Ending balance, value at Jun. 30, 2019	832,797	$ 34,668	316,268	481,861	0
Beginning balance, shares at Mar. 31, 2019		34,668
Beginning balance, value at Mar. 31, 2019	828,950	$ 34,668	316,268	478,014	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	15,548			15,548
Common stock dividends declared	(11,701)			(11,701)
Ending balance, shares at Jun. 30, 2019		34,668
Ending balance, value at Jun. 30, 2019	$ 832,797	$ 34,668	$ 316,268	$ 481,861	$ 0