Commitments and Contingencies (Details-1) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012
Air Quality Regulations [Abstract]
Minimum capacity to be regulated by CSAPR/CAIR - MW	25
CAIR phase one emission reductions for NOx	55.00%
CAIR phase one emission reductions for SO2	40.00%
CAIR phase two emission reductions for NOx	65.00%
CAIR phase two emission reductions for SO2	70.00%
Coal-fired electric generating units - MW requirement	150
Minimum amount of fleet-wide mercury reduction in Wisconsin - Phase two	90.00%
Columbia Environmental Project [Member]
Expected Capital Expenditures [Line Items]
MGE's share of expected capital expenditures	$ 140.0
Columbia Commitments [Abstract]
Columbia environmental project commitments	$ 89.1
Columbia Units [Member]
Columbia Units [Line Items]
Generating capability - MW	225
Columbia's share of MGE's net generating capability	29.00%
Ownership interest in jointly owned plant	22.00%
WPL ownership interest of Columbia	46.20%
WPSC ownership interest of Columbia	31.80%

Name:	mgee_AirQualityRegulationsAbstract
Namespace Prefix:	mgee_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The reduction of annual regional emission for NOx required under the first phase of the Clean Air Interstate Rule (CAIR).

+ References

No definition available.

+ Details

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

- Definition

The reduction of annual regional emission for SO2 required under the first phase of the Clean Air Interstate Rule (CAIR).

+ References

No definition available.

+ Details

Name:	mgee_CairPhaseOneEmissionReductionsForSo2
Namespace Prefix:	mgee_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The reduction of annual regional emission for NOx required under the second phase (beginning in 2015) of the Clean Air Interstate Rule (CAIR).

+ References

No definition available.

+ Details

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

- Definition

The reduction of annual regional emission for SO2 required under the second phase (beginning in 2015) of the Clean Air Interstate Rule (CAIR).

+ References

No definition available.

+ Details

Name:	mgee_CairPhaseTwoEmissionReductionsForSo2
Namespace Prefix:	mgee_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

A large coal-fired electric generating Unit is one that produces at least this amount of MW.

+ References

No definition available.

+ Details

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

- Details

Name:	mgee_ColumbiaCommitmentsAbstract
Namespace Prefix:	mgee_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Committed Columbia environmental project commitments.

+ References

No definition available.

+ Details

Name:	mgee_ColumbiaEnvironmentalProjectCommitments
Namespace Prefix:	mgee_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Expected capital expenditures to be incurred during construction of an asset.

+ References

No definition available.

+ Details

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

- Details

Name:	mgee_ExpectedCapitalExpendituresLineItems
Namespace Prefix:	mgee_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

MGE's portion of the total operating capacity (in MWs) of each of the two units in the jointly owned utility plant.

+ References

No definition available.

+ Details

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

- Definition

WPL's proportionate share of interest (percent) in Columbia, a jointly owned utility plant.

+ References

No definition available.

+ Details

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

- Definition

WPSC's proportionate share of interest (percent) in Columbia, a jointly owned utility plant.

+ References

No definition available.

+ Details

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

- Definition

Under phase two of the Wisconsin State Mercury Rule, beginning January 2015, large coal-fired electric generating units had to reduce mercury emissions by this minimum percent or choose a multi-pollutation reduction approach.

+ References

No definition available.

+ Details

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

- Definition

Electric generating units in Wisconsin that meet the minimum capacity (in MWs) are regulated by U.S Cross-State Air Pollution Rule (CSAPR) and Clean Air Interstate Rule (CAIR).

+ References

No definition available.

+ Details

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

- 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://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

Reference 2: 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

+ Details

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