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]
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle	$ 0			$ 377	$ (377)
Beginning balance, shares at Dec. 31, 2017		34,668
Beginning balance, value at Dec. 31, 2017	778,187	$ 34,668	$ 316,268	426,874	377
Beginning balance, value - Adjusted at Dec. 31, 2017	778,187			427,251	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	67,858			67,858
Common stock dividends declared	(34,062)			(34,062)
Ending balance, shares at Sep. 30, 2018		34,668
Ending balance, value at Sep. 30, 2018	811,983	$ 34,668	316,268	461,047	0
Beginning balance, shares at Jun. 30, 2018		34,668
Beginning balance, value at Jun. 30, 2018	794,178	$ 34,668	316,268	443,242	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	29,506			29,506
Common stock dividends declared	(11,701)			(11,701)
Ending balance, shares at Sep. 30, 2018		34,668
Ending balance, value at Sep. 30, 2018	811,983	$ 34,668	316,268	461,047	0
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)
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