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, 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
Beginning balance, shares at Dec. 31, 2020		36,163
Beginning balance, value at Dec. 31, 2020	976,000	$ 36,163	394,408	545,429	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	92,701			92,701
Common stock dividends declared	(40,774)			(40,774)
Issuance of common stock, net	0
Equity-based compensation plans and other	358		358
Ending balance, shares at Sep. 30, 2021		36,163
Ending balance, value at Sep. 30, 2021	1,028,285	$ 36,163	394,766	597,356	0
Beginning balance, shares at Jun. 30, 2021		36,163
Beginning balance, value at Jun. 30, 2021	1,007,301	$ 36,163	394,686	576,452	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	34,917			34,917
Common stock dividends declared	(14,013)			(14,013)
Equity-based compensation plans and other	80		80
Ending balance, shares at Sep. 30, 2021		36,163
Ending balance, value at Sep. 30, 2021	$ 1,028,285	$ 36,163	$ 394,766	$ 597,356	$ 0