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	20,001			20,001
Common stock dividends declared	(11,181)			(11,181)
Ending balance, shares at Mar. 31, 2018		34,668
Ending balance, value at Mar. 31, 2018	787,007	$ 34,668	316,268	436,071	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	24,007			24,007
Common stock dividends declared	(11,701)			(11,701)
Ending balance, shares at Mar. 31, 2019		34,668
Ending balance, value at Mar. 31, 2019	$ 828,950	$ 34,668	$ 316,268	$ 478,014	$ 0