Investments (Details-1) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale securities:
Cost basis		$ 6,662	$ 2,216
Gross unrealized gains		675	346
Gross unrealized losses		(46)	(8)
Fair Value		7,291	2,554
Equity method investments:
Equity method investments		60,426	73,616
Other Investments		55	120
Total investments		67,772	76,290
Results of Investment Liquidation [Abstract]
Proceeds from sale of investments		677	408	$ 19
Gain (loss) on sale of investments		522	121	10
ATC [Member] | Tax Cuts and Jobs Act [Member]
Equity method investments:
Equity method investments		(20,400)
ATC and ATC Holdco [Member]
Equity method investments:
Equity method investments	[1],[2]	59,298	72,458
Other Equity Method Investments [Member]
Equity method investments:
Equity method investments		1,128	1,158
MGE [Member]
Available-for-sale securities:
Cost basis		455	455
Gross unrealized gains		0	40
Gross unrealized losses		(46)	(8)
Fair Value		409	487
Equity method investments:
Equity method investments		0	0
Other Investments		0	0
Total investments		409	487
Results of Investment Liquidation [Abstract]
Proceeds from sale of investments		0	16	19
Gain (loss) on sale of investments		0	(8)	$ 10
MGE [Member] | ATC and ATC Holdco [Member]
Equity method investments:
Equity method investments		0	0
MGE [Member] | Other Equity Method Investments [Member]
Equity method investments:
Equity method investments		$ 0	$ 0

[1] In December 2017, there was a $ 20.4 million one-time tax impact related to ATC as a result of the Tax Act. See Footnote 12 for further information.

[2] MGE Transco holds an ownership interest in ATC, and MGEE Transco holds an ownership interest in ATC Holdco. In July 2016, MGE's ownership interest in MGE Transco declined below a majority, resulting in MGE Energy's investment in MGE Transco being deconsolidated from MGE's consolidated financial statements. See Footnote 8 for further discussion of noncontrolling interest. In December 2016, MGE's ownership interest in MGE Transco was transferred to MGE Energy, see "ATC and ATC Holdco" below f or additional information.