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Notes Payable to Banks, Commercial Paper, and Lines of Credit (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Short-term Debt [Line Items]      
Short-term debt outstanding $ 7,000,000us-gaap_ShortTermBorrowings [1] $ 0us-gaap_ShortTermBorrowings [1] $ 0us-gaap_ShortTermBorrowings [1]
Weighted-average interest rate 0.20%us-gaap_ShortTermDebtWeightedAverageInterestRate [1] 0.00%us-gaap_ShortTermDebtWeightedAverageInterestRate [1] 0.00%us-gaap_ShortTermDebtWeightedAverageInterestRate [1]
Maximum short-term borrowings 9,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod [1] 32,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod [1] 16,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod [1]
Average short-term borrowings 182,000us-gaap_ShorttermDebtAverageOutstandingAmount [1] 6,992,000us-gaap_ShorttermDebtAverageOutstandingAmount [1] 1,154,000us-gaap_ShorttermDebtAverageOutstandingAmount [1]
Weighted-average interest rate during the year 0.24%mgee_ShortTermDebtWeightedAverageInterestRateDuringPeriod [1] 0.18%mgee_ShortTermDebtWeightedAverageInterestRateDuringPeriod [1] 0.17%mgee_ShortTermDebtWeightedAverageInterestRateDuringPeriod [1]
Line of Credit [Member]      
Short-term Debt [Line Items]      
Available lines of credit 150,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		 [1] 150,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		 [1] 115,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		 [1]
MGE [Member]      
Short-term Debt [Line Items]      
Short-term debt outstanding 7,000,000us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 0us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 0us-gaap_ShortTermBorrowings
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Weighted-average interest rate 0.20%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 0.00%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 0.00%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Maximum short-term borrowings 9,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 32,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 16,000,000us-gaap_ShorttermDebtMaximumAmountOutstandingDuringPeriod
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Average short-term borrowings 182,000us-gaap_ShorttermDebtAverageOutstandingAmount
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 6,992,000us-gaap_ShorttermDebtAverageOutstandingAmount
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 1,154,000us-gaap_ShorttermDebtAverageOutstandingAmount
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
Weighted-average interest rate during the year 0.24%mgee_ShortTermDebtWeightedAverageInterestRateDuringPeriod
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 0.18%mgee_ShortTermDebtWeightedAverageInterestRateDuringPeriod
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember		 0.17%mgee_ShortTermDebtWeightedAverageInterestRateDuringPeriod
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
MGE [Member] | Line of Credit [Member]      
Short-term Debt [Line Items]      
Available lines of credit 100,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		 100,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		 75,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember
MGE [Member] | Line of Credit [Member] | MGE $100 million unsecured committed revolving line of credit      
Short-term Debt [Line Items]      
Available lines of credit 100,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= mgee_MgeOneHundredMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
Line of credit, borrowings outstanding 0us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= mgee_MgeOneHundredMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
Debt covenant, required parent company ownership of MGE 100.00%mgee_DebtInstrumentCovenantTermsRequiredParentCompanyOwnershipOfOutstandingVotingEquityInterestInSubsidiary
/ us-gaap_DebtInstrumentAxis
= mgee_MgeOneHundredMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
MGE [Member] | Line of Credit [Member] | MGE $100 million unsecured committed revolving line of credit | Minimum [Member]      
Short-term Debt [Line Items]      
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value 30.00%mgee_DebtInstrumentCovenantTermsOwnershipInterestByOutsidePartyResultingInRepurchaseOfNotesAtParValue
/ us-gaap_DebtInstrumentAxis
= mgee_MgeOneHundredMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
MGE [Member] | Line of Credit [Member] | MGE $100 million unsecured committed revolving line of credit | Maximum [Member]      
Short-term Debt [Line Items]      
Debt to total capitalization ratio 0.65us-gaap_RatioOfIndebtednessToNetCapital1
/ us-gaap_DebtInstrumentAxis
= mgee_MgeOneHundredMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
MGE Energy [Member] | Line of Credit [Member] | MGE Energy $50 million unsecured commiteed revolving line of credit      
Short-term Debt [Line Items]      
Available lines of credit 50,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= mgee_MgeEnergyFiftyMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
Line of credit, borrowings outstanding $ 0us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= mgee_MgeEnergyFiftyMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
Debt covenant, required parent company ownership of MGE 100.00%mgee_DebtInstrumentCovenantTermsRequiredParentCompanyOwnershipOfOutstandingVotingEquityInterestInSubsidiary
/ us-gaap_DebtInstrumentAxis
= mgee_MgeEnergyFiftyMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
MGE Energy [Member] | Line of Credit [Member] | MGE Energy $50 million unsecured commiteed revolving line of credit | Minimum [Member]      
Short-term Debt [Line Items]      
Debt covenant, ownership interest by outside party resulting in repurchase of notes at par value 30.00%mgee_DebtInstrumentCovenantTermsOwnershipInterestByOutsidePartyResultingInRepurchaseOfNotesAtParValue
/ us-gaap_DebtInstrumentAxis
= mgee_MgeEnergyFiftyMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
MGE Energy [Member] | Line of Credit [Member] | MGE Energy $50 million unsecured commiteed revolving line of credit | Maximum [Member]      
Short-term Debt [Line Items]      
Debt to total capitalization ratio 0.65us-gaap_RatioOfIndebtednessToNetCapital1
/ us-gaap_DebtInstrumentAxis
= mgee_MgeEnergyFiftyMillionUnsecuredCommittedRevolvingLineOfCreditMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LineOfCreditMember		    
[1]

MGE Energy short-term borrowings include MGE Energy and MGE lines of credit and MGE commercial paper.