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Regional Transmission Organizations (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Regional Transmission Organizations Disclosure [Abstract] |  |  |
Reduction to sales and purchased power expense from reporting RTO transactions from net basis reporting | $ 68.6 | $ 91.1 | $ 78.0

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- Definition 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. + References No definition available. + Details
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Namespace Prefix: | mgee_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | mgee_RegionalTransmissionOrganizationsDisclosureAbstract
Namespace Prefix: | mgee_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration