Investments (Details-1) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Available-for-sale securities:
Cost basis			$ 6,700
Gross unrealized gains			700
Gross unrealized losses			(100)
Equity Securities	[1]	$ 9,363	7,291
Equity method investments:
Equity method investments		66,339	60,426
Other Investments		2,298	55
Total investments		78,000	67,772
Results of Investment Liquidation [Abstract]
Proceeds from sale of investments		960	677		$ 408
Gain (loss) on sale of investments		476	522		121
Tax Cuts and Jobs Act [Member]
Equity method investments:
Equity method investments			20,375
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		66,313	59,298	[2]
Other Equity Method Investments [Member]
Equity method investments:
Equity method investments		26	1,128
MGE [Member]
Available-for-sale securities:
Cost basis			500
Gross unrealized losses			(100)
Equity Securities	[1]	388	409
Equity method investments:
Equity method investments		0	0
Other Investments		0	0
Total investments		388	409
Results of Investment Liquidation [Abstract]
Proceeds from sale of investments		3	0		16
Gain (loss) on sale of investments		3	0		$ (8)
MGE [Member] | Tax Cuts and Jobs Act [Member]
Equity method investments:
Equity method investments			0
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] Reflects modified retrospective application of new authoritative guidance related to Financial Instruments as described in Footnote 2. Prior to January 1, 2018, investments were considered available for sale securities. As of December 31, 2017, MGE Energy had available for sale securities with a cost basis of $6.7 million, gross unrealized gains of $0.7 million, and gross unrealized losses of less than $0.1 million. As of December 31, 2017, MGE had available for sale securities with a cost basis of $ 0.5 million and gross unrealized losses of less than $0.1 million.

[2] 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.