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Rate Matters (Details) - PSCW [Member] - MGE [Member] - USD ($) | 4 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2015 | Jun. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2015 | Jul. 31, 2016 | Jan. 31, 2016 | Aug. 31, 2015
Rate Proceedings [Abstract] |  |  |  |  |  |  |
Authorized return on equity, percentage |  |  |  | 10.20% |  |  |
Fuel Rules [Abstract] |  |  |  |  |  |  |
Fuel rules, bandwidth |  | 2.00% |  |  |  |  |
Fuel rules, electric fuel deferred costs upper threshold |  | 102.00% |  |  |  |  |
Fuel rules, electric fuel deferred costs lower threshold |  | 98.00% |  |  |  |  |
Deferred fuel rules monitored costs |  | $ 14,800,000 |  |  |  |  |
Electric Fuel Monitored Costs, 2016 [Member] |  |  |  |  |  |  |
Fuel Rules [Abstract] |  |  |  |  |  |  |
Change in fuel rules monitored costs |  |  |  |  |  | $ (14,800,000) |
Deferred fuel rules monitored costs |  | 2,300,000 |  |  |  |  |
Fuel Rules Credit ($0.00256/kWh), 2015/2016 [Member] |  |  |  |  |  |  |
Fuel Rules [Abstract] |  |  |  |  |  |  |
Electric fuel credit (per kWh) | $ 0.00256 | 0.00256 |  |  |  |  |
Electric fuel credit ($) |  |  |  |  |  |  | $ 10,900,000
Return of electric fuel credit, total | $ 2,600,000 | $ 3,900,000 |  |  |  |  |
Fuel Rules Refund, 2015/2016 [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Fuel Rules [Abstract] |  |  |  |  |  |  |
Electric fuel credit ($) |  |  |  |  | $ 15,700,000 |  |
Electric Rate Proceeding [Member] |  |  |  |  |  |  |
Rate Proceedings [Abstract] |  |  |  |  |  |  |
Authorized rate increase (decrease), percentage |  | 0.00% |  | 3.80% |  |  |
Authorized rate increase (decrease), amount |  | $ 0 |  | $ 15,400,000 |  |  |
Electric Rate Proceeding [Member] | Proposed Future Rate Matters [Member] |  |  |  |  |  |  |
Rate Proceedings [Abstract] |  |  |  |  |  |  |
Proposed rate increase (decrease), percentage |  |  | 1.70% |  |  |  |
Gas Rate Proceeding [Member] |  |  |  |  |  |  |
Rate Proceedings [Abstract] |  |  |  |  |  |  |
Authorized rate increase (decrease), percentage |  | 0.00% |  | (2.00%) |  |  |
Authorized rate increase (decrease), amount |  | $ 0 |  | $ (3,800,000) |  |  |
Gas Rate Proceeding [Member] | Proposed Future Rate Matters [Member] |  |  |  |  |  |  |
Rate Proceedings [Abstract] |  |  |  |  |  |  |
Proposed rate increase (decrease), percentage |  |  | 3.70% |  |  |  |

X
- Definition This amount represents the change in fuel rules monitored costs approved by the PSCW. + References No definition available. + Details
Name: | mgee_Changeinfuelrulesmonitoredcosts
Namespace Prefix: | mgee_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of electric fuel rules monitored costs which have been deferred outside the symmetrical cost tolerance band authorized by a regulatory agency. + References No definition available. + Details
Name: | mgee_Deferredfuelrulesmonitoredcosts
Namespace Prefix: | mgee_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of fuel savings approved by the PSCW for an electric fuel credit to be returned to customers via a bill credit. + References No definition available. + Details
Name: | mgee_Electricfuelcredit
Namespace Prefix: | mgee_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The approved amount ($/kWh) for an electric fuel credit to be returned to customers via a bill credit. + References No definition available. + Details
Name: | mgee_Electricfuelcreditperkwh
Namespace Prefix: | mgee_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | mgee_FuelRulesAbstract
Namespace Prefix: | mgee_
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Balance Type: | na
Period Type: | duration

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- Definition Pursuant to the provisions of the rate order, the fuel rules bandwidth will be plus or minus this percentage. + References No definition available. + Details
Name: | mgee_FuelRulesBandwidth
Namespace Prefix: | mgee_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Under fuel rules, the amount in which MGE is required to defer the benefit of lower costs if actual electric fuel costs were less than the allowed electric fuel costs in latest rate order. + References No definition available. + Details
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Namespace Prefix: | mgee_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Under fuel rules, the amount in which MGE would defer if actual electric fuel costs exceeded allowed electric fuel costs in latest rate order. + References No definition available. + Details
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Namespace Prefix: | mgee_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of electric fuel-related savings that has been returned in customer bill credits. + References No definition available. + Details
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Namespace Prefix: | mgee_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Percentage of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of public utility's approved return on equity. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
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- References No definition available. + Details
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- Definition Percentage of public utility's requested rate increase (decrease) with regulatory agency. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
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- Details
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- Details
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- Details
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Namespace Prefix: |
Data Type: | na
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- Details
Name: | mgee_FuelRulesCreditYearAxis=mgee_FuelRulesCredit20152016Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | mgee_FuelRulesCreditYearAxis=mgee_Fuelrulesrefund20152016memberMember
Namespace Prefix: |
Data Type: | na
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- Details
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
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- Details
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