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, 2016		34,668
Beginning balance, value at Dec. 31, 2016	$ 724,088	$ 34,668	$ 316,268	$ 372,950	$ 202
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income	19,300			19,300
Other comprehensive income (loss)	97				97
Common stock dividends declared	(10,660)			(10,660)
Ending balance, shares at Mar. 31, 2017		34,668
Ending balance, value at Mar. 31, 2017	732,825	$ 34,668	316,268	381,590	299
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
Cumulative effect of new accounting principle at Dec. 31, 2017	0			377	(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