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	26,037			26,037
Common stock dividends declared	(12,221)			(12,221)
Ending balance, shares at Mar. 31, 2020		34,668
Ending balance, value at Mar. 31, 2020	869,492	$ 34,668	316,268	518,556	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	34,933			34,933
Common stock dividends declared	(13,380)			(13,380)
Equity-based compensation plans and other	158		158
Ending balance, shares at Mar. 31, 2021		36,163
Ending balance, value at Mar. 31, 2021	$ 997,711	$ 36,163	$ 394,566	$ 566,982	$ 0