Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 152,357	$ 163,337
Operating expenses	(79,951)	(78,623)
Other income, net	62	388
Interest expense, net	(24,483)	(21,996)
Earnings before members' income taxes	$ 47,985	$ 63,106

The amount of interest reported by an equity method investment of the entity.

No definition available.

Name:	mgee_EquityMethodInvestmentSummarizedFinancialInformationInterestIncomeExpense
Namespace Prefix:	mgee_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of net income or net loss before taxes reported by an equity method investment of the entity.

+ References

No definition available.

+ Details

Name:	mgee_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLossBeforeTaxes
Namespace Prefix:	mgee_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of other income or expenses reported by an equity method investment of the entity.

+ References

No definition available.

+ Details

Name:	mgee_EquityMethodInvestmentSummarizedFinancialInformationOtherIncomeExpense
Namespace Prefix:	mgee_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of the cost of sales reported by an equity method investment of the entity.

+ References

No definition available.

+ Details

Name:	us-gaap_EquityMethodInvestmentSummarizedFinancialInformationCostOfSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

+ References

No definition available.

+ Details

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

- Details

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