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]	AOCI Attributable to Parent [Member]
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 (loss)	92,701			92,701
Common stock dividends declared	(40,774)			(40,774)
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 (loss)	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
Beginning balance, shares at Dec. 31, 2021		36,163
Beginning balance, value at Dec. 31, 2021	1,027,468	$ 36,163	394,903	596,402	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	89,901			89,901
Common stock dividends declared	(42,763)			(42,763)
Equity-based compensation plans and other	594		594
Ending balance, shares at Sep. 30, 2022		36,163
Ending balance, value at Sep. 30, 2022	1,075,200	$ 36,163	395,497	643,540	0
Beginning balance, shares at Jun. 30, 2022		36,163
Beginning balance, value at Jun. 30, 2022	1,056,057	$ 36,163	395,338	624,556	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	33,720			33,720
Common stock dividends declared	(14,736)			(14,736)
Equity-based compensation plans and other	159		159
Ending balance, shares at Sep. 30, 2022		36,163
Ending balance, value at Sep. 30, 2022	$ 1,075,200	$ 36,163	$ 395,497	$ 643,540	$ 0