Schedule I - Condensed Parent Company Finanical Statements (Details-Cash Flow Statement) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Flows Provided by Operating Activities	$ 132,991	$ 147,513	$ 141,185
Investing Activities:
Other	32	(209)	(230)
Cash Used for Investing Activities	(116,361)	(86,826)	(73,313)
Financing Activities:
Cash dividends paid on common stock	(43,682)	(41,775)	(40,043)
Other	(597)	(70)	(1,018)
Cash Used for Financing Activities	(4,637)	(46,112)	(52,243)
Change in cash and cash equivalents:	11,993	14,575	15,629
Cash and cash equivalents at beginning of period	95,959	81,384	65,755
Cash and cash equivalents at end of period	107,952	95,959	81,384
MGE Energy [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net Cash Flows Provided by Operating Activities	70,268	74,994	37,085
Investing Activities:
Contributions to affiliates	(6,522)	(2,789)	(3,360)
Contributions to other Investments	(4,534)	(360)	(330)
Other	843	385	0
Cash Used for Investing Activities	(10,213)	(2,764)	(3,690)
Financing Activities:
Cash dividends paid on common stock	(43,682)	(41,775)	(40,043)
Other	(58)	(11)	0
Cash Used for Financing Activities	(43,740)	(41,786)	(40,043)
Change in cash and cash equivalents:	16,315	30,444	(6,648)
Cash and cash equivalents at beginning of period	82,225	51,781	58,429
Cash and cash equivalents at end of period	$ 98,540	$ 82,225	$ 51,781