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Share Based Compensation (Details) (Performance Units [Member], USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 0 Months Ended 1 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Feb. 20, 2015
Jan. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense $ 0.5us-gaap_AllocatedShareBasedCompensationExpense $ 0.8us-gaap_AllocatedShareBasedCompensationExpense    
Cash payments distributed related to awards previously granted and now payable       1.3us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsTotalShareBasedLiabilitiesPaid
Awards forfeited during period, value 0.2mgee_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresInPeriod 0mgee_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresInPeriod    
Awards forfeited during period, units 4,676us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures    
Outstanding awards vested during period $ 5.3us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue      
Performance Unit Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Awards granted during period (in units)     18,948us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= mgee_PerformanceUnitPlanMember
 
Award vesting period 5 years      
Director Incentive Agreement [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Awards granted during period (in units)       3,794us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_PlanNameAxis
= mgee_DirectorIncentiveAgreementMember
Award vesting period 3 years