Regional Transmission Organizations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Regional Transmission Organizations Disclosure [Abstract]
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting	$ 91.1	$ 78.0	$ 75.9

The transactions for whichan entity buys and sells power within the same hour to meet electric energy delivery requirements are reported on a net basis. The treatment results in a reduction to sales to the market and purchased power expense.

No definition available.

Name:	mgee_RegionalTransmissionOrganizationReductionToSalesAndPurchasedPowerExpenseFromNetBasisReporting
Namespace Prefix:	mgee_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration