Schedule I - Condensed Parent Company Finanical Statements (Details-Comprehensive Income Statement) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Operating Expenses:
Other operations and maintenance										$ 173,629		$ 167,989	$ 164,478
Total Operating Expenses	$ 114,000		$ 96,538	$ 99,912	$ 123,913	$ 114,967	$ 90,190	$ 93,364	$ 117,996	434,363		416,517	439,773
Operating Income (Loss)	26,274		43,001	26,551	32,910	23,958	46,527	28,212	29,531	128,736		128,228	124,255
Equity earnings of investments										10,125		8,428	7,728
Income before income taxes										119,700		118,073	112,706
Income tax provision	13,393	[1]	(15,584)	(8,736)	(11,167)	(7,570)	(15,714)	(9,284)	(9,945)	(22,094)	[1]	(42,513)	(41,363)
Net Income	$ 36,134		$ 26,629	$ 15,543	$ 19,300	$ 14,502	$ 27,880	$ 16,150	$ 17,028	97,606		75,560	71,343
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract]
Unrealized gain (loss) on available for sale securities, net of tax										175		(155)	(101)
Comprehensive Income										97,781		75,405	71,242
MGE Energy [Member]
Operating Expenses:
Other operations and maintenance										880		720	690
Total Operating Expenses										880		720	690
Operating Income (Loss)										(880)		(720)	(690)
Equity earnings of investments										99,246		75,581	71,306
Other income/(loss), net										(2,165)		435	526
Interest income, net										605		176	136
Income before income taxes										96,806		75,472	71,278
Income tax provision										800		88	65
Net Income										97,606		75,560	71,343
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract]
Unrealized gain (loss) on available for sale securities, net of tax										175		(155)	(101)
Comprehensive Income										$ 97,781		$ 75,405	$ 71,242

[1] In December 2017, there was a $21.7 million one-time tax impact as a result of the Tax Act. See Footnote 12 for further information.