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Related Party Transactions - Summary of Cemig's Principal Balances and Transactions With Related Parties (Parenthetical) (Detail) R$ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Jan. 31, 2018 BRL (R$) | Dec. 31, 2017 BRL (R$) | Dec. 31, 2017 BRL (R$) Installment Tranche
Disclosure of transactions between related parties [line items] |  |  |
Due and unpaid payment of debits relating to supply of power |  |  | R$ 113
Number of installments | Installment |  |  | 24
Amount paid for first installment |  | R$ 5 |
Number of installments | Installment |  |  | 24
Cemig GT [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Advance payments |  |  | R$ 52
Sa Carvalho [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Advance payments |  |  | 12
Rosal [Member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Advance payments |  |  | 6
Renova [member] | Cemig GT [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Settlement amount receivables |  | 272 | R$ 272
Renova [member] | Two Thousand and Eighteen [Member] | Cemig GT [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Number of installments | Installment |  |  | 11
Settlement amount receivables |  | 60 | R$ 60
Number of installments | Installment |  |  | 11
Percentage of settlement amount receivable |  |  | 150.00%
Renova [member] | Two Thousand and Eighteen [Member] | Cemig GT [member] | Settlement agreement [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Settlement amount initial payment | R$ 6 |  |
Renova [member] | Two Thousand and Twenty [Member] | Cemig GT [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Number of installments | Installment |  |  | 12
Settlement amount receivables |  | 94 | R$ 94
Number of installments | Installment |  |  | 12
Percentage of settlement interest rate |  |  | 155.00%
Renova [member] | Two Thousand and Twenty One [Member] | Cemig GT [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Number of installments | Installment |  |  | 12
Settlement amount receivables |  | R$ 118 | R$ 118
Number of installments | Installment |  |  | 12
Percentage of settlement interest rate |  |  | 155.00%
Santo Antonio Energia [member] |  |  |
Disclosure of transactions between related parties [line items] |  |  |
Advance payments |  |  | R$ 70
Number of tranches | Tranche |  |  | 12

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