Subsequent Events (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
ATC Capital Contribution [Member]
Subsequent Events [Line Items]
Subsequent Event, Date	Jul. 29, 2011
Subsequent Event, Description	MGE Transco made a voluntary $0.3 million capital contribution to ATC
Subsequent Event, Amount	$ 0.3
MGE Energy [Member] | Credit Agreement Amendment [Member]
Subsequent Events [Line Items]
Subsequent Event, Revolving Credit Facility	40.0
MGE [Member] | Credit Agreement Amendment [Member]
Subsequent Events [Line Items]
Subsequent Event, Revolving Credit Facility	$ 75.0
Credit Agreement Amendment [Member]
Subsequent Events [Line Items]
Subsequent Event, Date	Aug. 01, 2011
Subsequent Event, Description	On August 1, 2011, each of MGE Energy and MGE entered into an amendment to its existing credit agreement dated as of July 30, 2010, with various financial institutions and JPMorgan Chase Bank, N.A., as administrative agent.
Subsequent Event, Adder LIBOR-Based Borrowings	0.85%
Subsequent Event, Amendment Fee	0.075%