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	34,843			34,843
Other comprehensive income (loss)	129				129
Common stock dividends declared	(21,321)			(21,321)
Ending balance, shares at Jun. 30, 2017		34,668
Ending balance, value at Jun. 30, 2017	737,739	$ 34,668	316,268	386,472	331
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	38,352			38,352
Common stock dividends declared	(22,361)			(22,361)
Ending balance, shares at Jun. 30, 2018		34,668
Ending balance, value at Jun. 30, 2018	$ 794,178	$ 34,668	$ 316,268	$ 443,242	$ 0