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Loans, Financings and Debentures - Consolidated Totals of Funds Raised (Parenthetical) (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 14, 2017
Dec. 31, 2017
Disclosure of detailed information about borrowings [line items]    
Taxes with no cash effect   R$ 10
Non-convertible debentures amortisation period 36 months  
Cemig Telecom [member]    
Disclosure of detailed information about borrowings [line items]    
Non-convertible debentures maturity period 4 years 6 months  
Annual remuneration percentage 146.50%