Joint Plant Ownership (Details) (USD $)	12 Months Ended
Dec. 31, 2014 MW	Dec. 31, 2013	Dec. 31, 2012
Columbia Units [Member]
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	$ 268,597,000	$ 123,097,000
Accumulated depreciation	(80,645,000)	(78,880,000)
Property, plant, and equipment, net	187,952,000	44,217,000
Construction work in progress	6,941,000	120,858,000
Total property, plant, and equipment	194,893,000	165,075,000
Columbia Units [Member] \| MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, proportion of total net summer generating capability	31.00%
Jointly owned utility plant, plant capacity (in MW)	239
Jointly owned utility plant, ownership interest	22.00%
Jointly owned utility plant, fuel, operating, and maintenance expense	28,100,000	37,500,000	36,300,000
Elm Road Units [Member] \| MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, fuel, operating, and maintenance expense	20,300,000	13,400,000	13,200,000
Elm Road Units [Member] \| MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, proportion of total net summer generating capability	14.00%
Jointly owned utility plant, proportion of total net summer generating capability, plant capacity (in MW)	106
Jointly owned utility plant, number of operating units	2
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	199,582,000	198,198,000
Accumulated depreciation	(19,962,000)	(15,541,000)
Property, plant, and equipment, net	179,620,000	182,657,000
Construction work in progress	1,976,000	1,115,000
Total property, plant, and equipment	181,596,000	183,772,000
Elm Road Unit 1 [Member] \| MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW)	615
Jointly owned utility plant, ownership interest	8.33%
Elm Road Unit 2 [Member] \| MGE Power Elm Road [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, plant capacity (in MW)	615
Jointly owned utility plant, ownership interest	8.33%
West Campus [Member] \| UW [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	45.00%
Jointly owned utility plant, fuel, operating, and maintenance expense	2,800,000	4,900,000	4,900,000
West Campus [Member] \| MGE Power West Campus [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	55.00%
Jointly Owned Plants, Net Plant Ownership [Abstract]
Plant	111,453,000	111,268,000
Accumulated depreciation	(24,691,000)	(21,704,000)
Property, plant, and equipment, net	$ 86,762,000	$ 89,564,000

Discloses entity portion of the fuel, operating, and maintenance expenses for the period.

No definition available.

Name:	mgee_JointlyOwnedUtilityPlantFuelOperatingAndMaintenanceExpense
Namespace Prefix:	mgee_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The total period end amount of plant in service net of accumulated depreciation for the utility's share of jointly owned electricity generation plant.

+ References

No definition available.

+ Details

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

- Definition

The total operating capacity of each of the units in the jointly owned utility plant.

+ References

No definition available.

+ Details

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

- Definition

A jointly owned utility plant's share of MGE's net generating capability.

+ References

No definition available.

+ Details

Name:	mgee_JointlyOwnedUtilityPlantProportionOfTotalNetSummerGeneratingCapability
Namespace Prefix:	mgee_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

A jointly owned utility plant's share of MGE's net generating capability capacity.

+ References

No definition available.

+ Details

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

- Definition

Number of operating units in the jointly owned utility plant.

+ References

No definition available.

+ Details

Name:	mgee_JointlyOwnedUtiliytPlantNumberOfOperatingUnits
Namespace Prefix:	mgee_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-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=6500719&loc=d3e659983-123033

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

+ Details

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

- Details

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

- Definition

The total period end amount of plant in service and construction in progress net of accumulated depreciation for the utility's share of jointly owned electricity generation plant.

+ References

+ Details

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

- Details

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

- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

+ Details

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

- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantOwnershipAmountOfPlantAccumulatedDepreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

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

- Details

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

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=mgee_ElmRoadUnit1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=mgee_ElmRoadUnit2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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