Joint Plant Ownership (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($) MW
Two Creeks Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
Projected Solar Farm Project Costs	$ 65.0
Public Utilities Property Plant And Equipment Construction Work In Progress	44.2
Allowance for Funds Used During Construction
AFUDC - equity funds	$ 0.8
Two Creeks Units [Member] \| MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	33.00%
Jointly owned utility plant, plant capacity (in MW) \| MW	150
Two Creeks Units [Member] \| MGE [Member] \| Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate - Signficiant projects (100%)	100.00%
Badger Hollow Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
Projected Solar Farm Project Costs	$ 65.0
Public Utilities Property Plant And Equipment Construction Work In Progress	22.3
Allowance for Funds Used During Construction
AFUDC - equity funds	$ 0.4
Badger Hollow Units [Member] \| MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	33.00%
Jointly owned utility plant, plant capacity (in MW) \| MW	150
Badger Hollow Units [Member] \| MGE [Member] \| Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate - Signficiant projects (100%)	100.00%

The total operating capacity (in megawatts) of the jointly owned utility plant.

No definition available.

Name:	mgee_Jointlyownedutilityplantplantcapacity
Namespace Prefix:	mgee_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

- Definition

The Company's projected costs for the current solar projects.

+ References

No definition available.

+ Details

Name:	mgee_Projectedsolarfarmprojectcosts
Namespace Prefix:	mgee_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction on 100 percent of Construction Work in Progress. This PSCW authorized rate is on significant projects. Projects that fit this description are: Saratoga Wind, Badger Hollow I, Two Creeks, its customer information billing project, and on certain environmental costs for Columbia.

+ References

No definition available.

+ Details

Name:	mgee_PublicUtilitiesAllowanceForFundsUsedDuringConstructionRateOneHundredPercent
Namespace Prefix:	mgee_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantInterestsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The component of the allowance for funds used during construction during the period based on an assumed rate of return on equity funds used in financing the construction of regulated assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501662&loc=d3e56162-110433

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 835
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501569&loc=d3e55921-110430

+ Details

Name:	us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionNetIncreaseAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period end amount of construction work in progress in public utility.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesPropertyPlantAndEquipmentConstructionWorkInProgress
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=mgee_TwocreeksunitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=mgee_MgeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryAgencyAxis=mgee_PublicServiceCommissionOfWisconsinMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=mgee_BadgerHollowUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: