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Derivative and Hedging Instruments (Details-4) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Regulatory Asset [Member] |  |
Change In Derivative Fair Value [Roll Forward] |  |
Beginning balance, | $ 54,998 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember | $ 63,893 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember
Change in unrealized loss (gain) | 1,318 us-gaap_UnrealizedGainLossOnDerivatives / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember | (13,601)us-gaap_UnrealizedGainLossOnDerivatives / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember
Realized gain (loss) reclassified to a deferred account | (1,061)mgee_DerivativeRealizedGainLossDeferred / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember | 1,535 mgee_DerivativeRealizedGainLossDeferred / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember
Realized gain (loss) reclassified to income statement | (1,289)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember | 6,386 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember
Ending balance, | 53,966 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember | 58,213 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = mgee_RegulatoryAssetMember
Other Current Assets [Member] |  |
Change In Derivative Fair Value [Roll Forward] |  |
Beginning balance, | 1,001 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember | 411 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember
Change in unrealized loss (gain) | 0 us-gaap_UnrealizedGainLossOnDerivatives / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember | 0 us-gaap_UnrealizedGainLossOnDerivatives / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember
Realized gain (loss) reclassified to a deferred account | 1,061 mgee_DerivativeRealizedGainLossDeferred / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember | (1,535)mgee_DerivativeRealizedGainLossDeferred / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember
Realized gain (loss) reclassified to income statement | (1,794)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember | 1,475 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember
Ending balance, | $ 268 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember | $ 351 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember

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- Definition Change in realized derivative gains and losses deferred. + References No definition available. + Details
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- Definition Fair value of the assets less the liabilities of a derivative or group of derivatives. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 + Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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