Madison Gas and Electric Company Consolidated Statements of Common Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	MGE [Member]	Common Stock [Member]	Common Stock [Member] MGE [Member]	Additional Paid-In Capital [Member] MGE [Member]	Retained Earnings [Member]	Retained Earnings [Member] MGE [Member]	Accumulated Other Comprehensive Income/(Loss) [Member]	Accumulated Other Comprehensive Income/(Loss) [Member] MGE [Member]	Noncontrolling Interest [Member] MGE [Member]
Beginning balance, shares at Dec. 31, 2016			34,668	17,348
Beginning balance, value at Dec. 31, 2016		$ 602,749		$ 17,348	$ 192,417		$ 277,300		$ 19	$ 115,665
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income		57,686					41,462			16,224
Other comprehensive income (loss)	$ 189	(38)						$ 189	(38)
Cash dividends paid to parent by MGE		(35,000)					(35,000)
Distributions to parent from noncontrolling interest		(16,500)								(16,500)
Ending balance, shares at Sep. 30, 2017			34,668	17,348
Ending balance, value at Sep. 30, 2017		608,897		$ 17,348	192,417		283,762		(19)	115,389
Beginning balance, shares at Dec. 31, 2017			34,668	17,348
Beginning balance, value at Dec. 31, 2017		632,774		$ 17,348	192,417		282,135		(28)	140,902
Cumulative effect of new accounting principle at Dec. 31, 2017	0	0				$ 377	(28)	(377)	28
Beginning balance, value - Adjusted at Dec. 31, 2017	$ 778,187	632,774				$ 427,251	282,107	$ 0	0
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Net Income		62,775					45,835			16,940
Distributions to parent from noncontrolling interest		(19,000)								(19,000)
Ending balance, shares at Sep. 30, 2018			34,668	17,348
Ending balance, value at Sep. 30, 2018		$ 676,549		$ 17,348	$ 192,417		$ 327,942		$ 0	$ 138,842