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Schedule I - Condensed Parent Company Finanical Statements (Details-Comprehensive Income Statement Parentheticals) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Condensed Income Statements, Captions [Line Items] |  |  |
Unrealized gain (loss) on available-for-sale securities, taxes expense (benefit) | $ 0 | $ 0 | $ 117
MGE Energy [Member] |  |  |
Condensed Income Statements, Captions [Line Items] |  |  |
Unrealized gain (loss) on available-for-sale securities, taxes expense (benefit) | $ 0 | $ 0 | $ 117

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- Definition Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e640-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669646-108580 + Details
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