Fair Value of Financial Instruments (Details-1) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Maximum [Member]	Sep. 30, 2013 Minimum [Member]	Sep. 30, 2013 Carrying Amount [Member]		Dec. 31, 2012 Carrying Amount [Member]		Sep. 30, 2013 Fair Value [Member]		Dec. 31, 2012 Fair Value [Member]		Sep. 30, 2013 MGE [Member] Carrying Amount [Member]		Dec. 31, 2012 MGE [Member] Carrying Amount [Member]		Sep. 30, 2013 MGE [Member] Fair Value [Member]		Dec. 31, 2012 MGE [Member] Fair Value [Member]
Assets:
Cash and cash equivalents				$ 75,330		$ 46,357		$ 75,330		$ 46,357		$ 42,590		$ 6,350		$ 42,590		$ 6,350
Liabilities:
Long-term debt				$ 404,805	[1]	$ 361,805	[1]	$ 445,247	[1]	$ 427,456	[1]	$ 404,805	[1]	$ 361,805	[1]	$ 445,247	[1]	$ 427,456	[1]
Fair Value Inputs Assets Quantitative Information [Line Items]
Basis adjustment - on peak	99.70%
Basis adjustment - off peak	97.90%
Counterparty fuel mix [Abstract]
Internal generation		70.00%	50.00%
Purchased power		50.00%	30.00%

[1] *Includes long-term debt due within one year.