Subsequent Events (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended						4 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 MGE Transco [Member] ATC [Member]	Dec. 31, 2011 MGE Transco [Member] ATC [Member]	Dec. 31, 2010 MGE Transco [Member] ATC [Member]	Jan. 31, 2013 MGE Transco [Member] ATC [Member] ATC Capital Contribution [Member]
Subsequent Events [Line Items]
Subsequent Event, Description							On January 31, 2013, MGE Transco made a voluntary $0.2 million capital contribution to ATC.
Contributions to ATC	$ 2,419	$ 1,008	$ 810	$ 2,100	$ 900	$ 700	$ 200