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Related Party Transactions - Summary of Financial Investments in Securities of Related Parties, in the Investment Fund (Parenthetical) (Detail) - Related parties [member] | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Investment [Line Items] |  |
Percentage of investment | 72.28% | 78.31%
Cemig [member] |  |
Investment [Line Items] |  |
Percentage of investment | 4.17% | 10.12%
Cemig GT [member] |  |
Investment [Line Items] |  |
Percentage of investment | 26.85% | 20.86%
Cemig D [member] |  |
Investment [Line Items] |  |
Percentage of investment | 19.90% | 24.94%
Other Subsidiaries [Member] |  |
Investment [Line Items] |  |
Percentage of investment | 21.36% | 22.39%

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- Definition Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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- Definition Percentage of investments + References No definition available. + Details
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