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Concession Financial Assets and Liabilities - Summary of Indemnity Values (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Financial Assets and Liabilities Of The Concession [Line Items]    
Adjustment of the BRR of Transmission Assets - Aneel Technical Note 183/2017 R$ 149  
Cemig GT [member] | Transmission Indemnity receivable [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [Line Items]    
Adjustment of the BRR of Transmission Assets - Aneel Technical Note 183/2017 149  
Regulatory Remuneration Base (BRR) - Dispatch 2,181 2016   R$ 1,177
Updating per MME 121 913
Monetary adjustment 103  
Amounts received (250) (285)
Net residual value of the assets to be received   892
Total indemnity R$ 1,928 R$ 1,805