Quarterly Summary of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Revenues:
Regulated electric revenues	$ 86,431	$ 112,869	$ 96,697	$ 98,852	$ 93,781	$ 119,836	$ 96,846	$ 93,494	$ 394,849	$ 403,957	$ 392,365
Regulated gas revenues	58,385us-gaap_GasDomesticRegulatedRevenue	21,404us-gaap_GasDomesticRegulatedRevenue	31,218us-gaap_GasDomesticRegulatedRevenue	110,713us-gaap_GasDomesticRegulatedRevenue	60,089us-gaap_GasDomesticRegulatedRevenue	18,864us-gaap_GasDomesticRegulatedRevenue	30,042us-gaap_GasDomesticRegulatedRevenue	72,467us-gaap_GasDomesticRegulatedRevenue	221,720us-gaap_GasDomesticRegulatedRevenue	181,462us-gaap_GasDomesticRegulatedRevenue	139,727us-gaap_GasDomesticRegulatedRevenue
Nonregulated revenues	891us-gaap_UnregulatedOperatingRevenue	862us-gaap_UnregulatedOperatingRevenue	850us-gaap_UnregulatedOperatingRevenue	680us-gaap_UnregulatedOperatingRevenue	1,393us-gaap_UnregulatedOperatingRevenue	1,399us-gaap_UnregulatedOperatingRevenue	1,400us-gaap_UnregulatedOperatingRevenue	1,276us-gaap_UnregulatedOperatingRevenue	3,283us-gaap_UnregulatedOperatingRevenue	5,468us-gaap_UnregulatedOperatingRevenue	9,231us-gaap_UnregulatedOperatingRevenue
Total Operating Revenues	145,707	135,135	128,765	210,245	155,263	140,099	128,288	167,237	619,852	590,887	541,323
Operating expenses	116,141us-gaap_UtilitiesOperatingExpense	95,015us-gaap_UtilitiesOperatingExpense	104,324us-gaap_UtilitiesOperatingExpense	166,274us-gaap_UtilitiesOperatingExpense	129,301us-gaap_UtilitiesOperatingExpense	100,199us-gaap_UtilitiesOperatingExpense	103,791us-gaap_UtilitiesOperatingExpense	129,609us-gaap_UtilitiesOperatingExpense	481,754us-gaap_UtilitiesOperatingExpense	462,900us-gaap_UtilitiesOperatingExpense	428,560us-gaap_UtilitiesOperatingExpense
Operating Income (Loss)	29,566us-gaap_OperatingIncomeLoss	40,120us-gaap_OperatingIncomeLoss	24,441us-gaap_OperatingIncomeLoss	43,971us-gaap_OperatingIncomeLoss	25,962us-gaap_OperatingIncomeLoss	39,900us-gaap_OperatingIncomeLoss	24,497us-gaap_OperatingIncomeLoss	37,628us-gaap_OperatingIncomeLoss	138,098us-gaap_OperatingIncomeLoss	127,987us-gaap_OperatingIncomeLoss	112,763us-gaap_OperatingIncomeLoss
Interest and other income, net	(5,780)mgee_InterestExpenseAndOtherIncomeNet	(2,505)mgee_InterestExpenseAndOtherIncomeNet	(1,320)mgee_InterestExpenseAndOtherIncomeNet	11mgee_InterestExpenseAndOtherIncomeNet	(4,056)mgee_InterestExpenseAndOtherIncomeNet	(855)mgee_InterestExpenseAndOtherIncomeNet	(1,946)mgee_InterestExpenseAndOtherIncomeNet	(1,366)mgee_InterestExpenseAndOtherIncomeNet
Income tax provision	(8,600)us-gaap_IncomeTaxExpenseBenefit	(14,286)us-gaap_IncomeTaxExpenseBenefit	(9,034)us-gaap_IncomeTaxExpenseBenefit	(16,265)us-gaap_IncomeTaxExpenseBenefit	(7,829)us-gaap_IncomeTaxExpenseBenefit	(14,692)us-gaap_IncomeTaxExpenseBenefit	(8,660)us-gaap_IncomeTaxExpenseBenefit	(13,678)us-gaap_IncomeTaxExpenseBenefit	(48,185)us-gaap_IncomeTaxExpenseBenefit	(44,859)us-gaap_IncomeTaxExpenseBenefit	(38,919)us-gaap_IncomeTaxExpenseBenefit
Net Income	$ 15,186us-gaap_NetIncomeLoss	$ 23,329us-gaap_NetIncomeLoss	$ 14,087us-gaap_NetIncomeLoss	$ 27,717us-gaap_NetIncomeLoss	$ 14,077us-gaap_NetIncomeLoss	$ 24,353us-gaap_NetIncomeLoss	$ 13,891us-gaap_NetIncomeLoss	$ 22,584us-gaap_NetIncomeLoss	$ 80,319us-gaap_NetIncomeLoss	$ 74,905us-gaap_NetIncomeLoss	$ 64,446us-gaap_NetIncomeLoss
Earnings per common share	$ 0.44us-gaap_EarningsPerShareBasicAndDiluted	$ 0.67us-gaap_EarningsPerShareBasicAndDiluted	$ 0.41us-gaap_EarningsPerShareBasicAndDiluted	$ 0.8us-gaap_EarningsPerShareBasicAndDiluted	$ 0.41us-gaap_EarningsPerShareBasicAndDiluted	$ 0.7us-gaap_EarningsPerShareBasicAndDiluted	$ 0.4us-gaap_EarningsPerShareBasicAndDiluted	$ 0.65us-gaap_EarningsPerShareBasicAndDiluted	$ 2.32us-gaap_EarningsPerShareBasicAndDiluted	$ 2.16us-gaap_EarningsPerShareBasicAndDiluted	$ 1.86us-gaap_EarningsPerShareBasicAndDiluted
Dividends per share	$ 0.283	$ 0.283	$ 0.272	$ 0.272	$ 0.272	$ 0.272	$ 0.263	$ 0.263	$ 1.11	$ 1.07	$ 1.04

he amount of interest expense and other income recognized during the period, net.

No definition available.

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

- Definition

Aggregate dividends paid during the period for each share of common stock outstanding.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_CommonStockDividendsPerShareCashPaid
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References

No definition available.

+ Details

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of electricity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(b))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

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

- Definition

Revenue derived from the regulated (by a federal, state, or local government or agency) generation, transmission and distribution of [natural] gas.

+ References

+ Details

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

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Article 4

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Income Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6515339

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

+ Details

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

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Net Income

-URI http://asc.fasb.org/extlink&oid=6518256

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.19)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.18)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.22)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 19

-Article 5

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 20

-Article 9

+ Details

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

- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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

- Definition

The total amount of operating revenues recognized during the period.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

The amount of unregulated operating revenues recognized during the period.

+ References

No definition available.

+ Details

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

- Definition

Amount of operating expense of regulated operation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.2(b))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

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