Investments (Details-2) (USD $) In Thousands, unless otherwise specified	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 30, 2015
Schedule Of Equity Method Investments [Line Items]
Equity in earnings in ATC	$ 9,150	$ 9,434	$ 9,079
Dividend income from ATC	7,740	7,404	7,146
Contributions to investments	2,185	1,660	2,419
MGE Transco [Member] | ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity in earnings in ATC	9,150	9,434	9,079
Dividend income from ATC	7,740	7,404	7,146
Contributions to investments	1,775	1,420	2,131
MGE Transco ownership interest in ATC	3.60%	3.60%
MGE Transco [Member] | ATC [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Contributions to investments				$ 200