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Pension Plans and Other Postretirement Benefits (Details-7) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Expected Cash Flows      
Expected employer contributions, 2015 $ 10,000,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear    
Expected employer contributions, 2016 0mgee_DefinedBenefitPlansExpectedFutureEmployerContributionsYearTwo    
Cash contributions to pension and other postretirement plans 3,321,000us-gaap_PensionAndOtherPostretirementBenefitContributions 34,765,000us-gaap_PensionAndOtherPostretirementBenefitContributions 28,857,000us-gaap_PensionAndOtherPostretirementBenefitContributions
Pension Benefits [Member]      
Benefit Payments      
Defined benefit plan expected future benefit payments, 2015 11,944,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2016 12,631,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2017 13,408,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2018 14,613,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2019 15,559,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2020-2024 92,915,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Postretirement Benefits [Member]      
Benefit Payments      
Defined benefit plan expected future benefit payments, 2015 2,890,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2016 3,101,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2017 3,499,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2018 3,990,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2019 4,470,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Defined benefit plan expected future benefit payments, 2020-2024 27,866,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Gross Postretirement Benefits [Member]      
Benefit Payments      
Defined benefit plan expected future benefit payments, 2015 3,101,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_GrossPostretirementBenefitsMember
   
Defined benefit plan expected future benefit payments, 2016 3,338,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_GrossPostretirementBenefitsMember
   
Defined benefit plan expected future benefit payments, 2017 3,757,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_GrossPostretirementBenefitsMember
   
Defined benefit plan expected future benefit payments, 2018 4,272,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_GrossPostretirementBenefitsMember
   
Defined benefit plan expected future benefit payments, 2019 4,776,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_GrossPostretirementBenefitsMember
   
Defined benefit plan expected future benefit payments, 2020-2024 29,871,000us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_GrossPostretirementBenefitsMember
   
Expected Medicare Part D Subsidy [Member]      
Benefit Payments      
Defined benefit plan expected future benefit payments, 2015 (211,000)us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_ExpectedMedicarePartDSubsidyMember
   
Defined benefit plan expected future benefit payments, 2016 (237,000)us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_ExpectedMedicarePartDSubsidyMember
   
Defined benefit plan expected future benefit payments, 2017 (258,000)us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_ExpectedMedicarePartDSubsidyMember
   
Defined benefit plan expected future benefit payments, 2018 (282,000)us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_ExpectedMedicarePartDSubsidyMember
   
Defined benefit plan expected future benefit payments, 2019 (306,000)us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_ExpectedMedicarePartDSubsidyMember
   
Defined benefit plan expected future benefit payments, 2020-2024 $ (2,005,000)us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mgee_ExpectedMedicarePartDSubsidyMember