MGE Energy Inc Consolidated Statements of Common Equity - USD ($) $ in Thousands	Total	Cumulative Effect Period Of Adoption Adjustment [Member]	Cumulative Effect Period Of Adoption Adjusted Balance [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect Period Of Adoption Adjustment [Member]	Retained Earnings [Member] Cumulative Effect Period Of Adoption Adjusted Balance [Member]	Accumulated Other Comprehensive (Loss)/ Income [Member]	Accumulated Other Comprehensive (Loss)/ Income [Member] Cumulative Effect Period Of Adoption Adjustment [Member]	Accumulated Other Comprehensive (Loss)/ Income [Member] Cumulative Effect Period Of Adoption Adjusted Balance [Member]
Beginning balance, shares at Dec. 31, 2017				34,668,000
Beginning balance, value at Dec. 31, 2017	$ 778,187	$ 0	$ 778,187	$ 34,668	$ 316,268	$ 426,874	$ 377	$ 427,251	$ 377	$ (377)	$ 0
Increase (Decrease) in Equity [Roll Forward]
Net Income	84,219					84,219
Common stock dividends declared	(45,762)					(45,762)
Issuance of common stock, net	0
Ending balance, shares at Dec. 31, 2018				34,668,000
Ending balance, value at Dec. 31, 2018	816,644			$ 34,668	316,268	465,708			0
Increase (Decrease) in Equity [Roll Forward]
Net Income	86,874					86,874
Common stock dividends declared	(47,842)					(47,842)
Issuance of common stock, net	0
Ending balance, shares at Dec. 31, 2019				34,668,000
Ending balance, value at Dec. 31, 2019	855,676			$ 34,668	316,268	504,740			0
Increase (Decrease) in Equity [Roll Forward]
Net Income	92,418					92,418
Common stock dividends declared	$ (51,729)					(51,729)
Common stock issued during period	1,500,000
Common stock issued, net value				1,495
Issuance of common stock, net	$ 79,635			$ 1,495	78,140
Ending balance, shares at Dec. 31, 2020				36,163,000
Ending balance, value at Dec. 31, 2020	$ 976,000			$ 36,163	$ 394,408	$ 545,429			$ 0