Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 MGE [Member]	Mar. 31, 2012 MGE [Member]	Apr. 18, 2013 Unrecognized Tax Benefit [Member]
Unrecognized tax benefits that would impact effective tax rate [Abstract]
Total unrecognized tax benefit	$ 3.4		$ 3.2
Subsequent Events [Line Items]
Subsequent Event, Description						On April 18, 2013, MGE Energy received notification from the Internal Revenue Service that the examination of tax periods 2007-2009 has been completed. Unrecognized tax benefits of $1.5 million related to those years will be reversed (in the second quarter) due to those positions not being adversely adjusted in the examination.
Unrecognized Tax Benefits, decreases resulting from settlements with taxing authorities						$ 1.5
Effective income tax rate [Line Items]
Effective income tax rate	37.70%	38.10%		37.70%	38.10%