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]
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	70,212			70,212
Common stock dividends declared	(35,622)			(35,622)
Issuance of common stock, net	0
Ending balance, shares at Sep. 30, 2019		34,668
Ending balance, value at Sep. 30, 2019	851,234	$ 34,668	316,268	500,298	0
Beginning balance, shares at Jun. 30, 2019		34,668
Beginning balance, value at Jun. 30, 2019	832,797	$ 34,668	316,268	481,861	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	30,657			30,657
Common stock dividends declared	(12,220)			(12,220)
Ending balance, shares at Sep. 30, 2019		34,668
Ending balance, value at Sep. 30, 2019	851,234	$ 34,668	316,268	500,298	0
Beginning balance, shares at Dec. 31, 2019		34,668
Beginning balance, value at Dec. 31, 2019	855,676	$ 34,668	316,268	504,740	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	76,622			76,622
Common stock dividends declared	$ (38,349)			(38,349)
Common stock issued during period	1,500	1,495
Issuance of common stock, net	$ 79,635	$ 1,495	78,140
Ending balance, shares at Sep. 30, 2020		36,163
Ending balance, value at Sep. 30, 2020	973,584	$ 36,163	394,408	543,013	0
Beginning balance, shares at Jun. 30, 2020		36,163
Beginning balance, value at Jun. 30, 2020	955,171	$ 36,163	394,408	524,600	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	31,794			31,794
Common stock dividends declared	(13,381)			(13,381)
Ending balance, shares at Sep. 30, 2020		36,163
Ending balance, value at Sep. 30, 2020	$ 973,584	$ 36,163	$ 394,408	$ 543,013	$ 0