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Related Party Transactions - Summary of Financial Investments in Securities of Related Parties, in the Investment Fund (Detail) - Related parties [member] - Investment | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Investment [Line Items] |  |
Number Of Investments | 21 | 12
ETAU [member] |  |
Investment [Line Items] |  |
Type | Debentures | Debentures
Annual contractual conditions | 108.00% of CDI | 108.00% of CDI
Maturity | Dec. 01, 2019 | Dec. 01, 2019
Number Of Investments | 7 | 8
Light [member] |  |
Investment [Line Items] |  |
Type | Promissory Note |
Annual contractual conditions | CDI + 3.50% |
Maturity | Jan. 22, 2019 |
Number Of Investments | 14 |
Axxiom [member] |  |
Investment [Line Items] |  |
Type |  | Debentures
Annual contractual conditions |  | 109.00% of CDI Rate
Maturity |  | Jan. 29, 2017
Number Of Investments |  | 4
Cemig [member] |  |
Investment [Line Items] |  |
Number Of Investments | 1 | 2
Cemig [member] | ETAU [member] |  |
Investment [Line Items] |  |
Number Of Investments |  | 1
Cemig [member] | Light [member] |  |
Investment [Line Items] |  |
Number Of Investments | 1 |
Cemig [member] | Axxiom [member] |  |
Investment [Line Items] |  |
Number Of Investments |  | 1
Cemig GT [member] |  |
Investment [Line Items] |  |
Number Of Investments | 8 | 3
Cemig GT [member] | ETAU [member] |  |
Investment [Line Items] |  |
Number Of Investments | 3 | 2
Cemig GT [member] | Light [member] |  |
Investment [Line Items] |  |
Number Of Investments | 5 |
Cemig GT [member] | Axxiom [member] |  |
Investment [Line Items] |  |
Number Of Investments |  | 1
Cemig D [member] |  |
Investment [Line Items] |  |
Number Of Investments | 6 | 4
Cemig D [member] | ETAU [member] |  |
Investment [Line Items] |  |
Number Of Investments | 2 | 3
Cemig D [member] | Light [member] |  |
Investment [Line Items] |  |
Number Of Investments | 4 |
Cemig D [member] | Axxiom [member] |  |
Investment [Line Items] |  |
Number Of Investments |  | 1
Other Subsidiaries [Member] |  |
Investment [Line Items] |  |
Number Of Investments | 6 | 3
Other Subsidiaries [Member] | ETAU [member] |  |
Investment [Line Items] |  |
Number Of Investments | 2 | 2
Other Subsidiaries [Member] | Light [member] |  |
Investment [Line Items] |  |
Number Of Investments | 4 |
Other Subsidiaries [Member] | Axxiom [member] |  |
Investment [Line Items] |  |
Number Of Investments |  | 1

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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 The maturity date of the investment. + 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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