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Concession Financial and Sector Assets and Liabilities - Additional Information (Detail)
R$ in Millions
 12 Months Ended
Aug. 31, 2018
BRL (R$)
Dec. 31, 2018
BRL (R$)
Plant
Dec. 31, 2017
BRL (R$)
Dec. 31, 2016
BRL (R$)
Aug. 16, 2016
BRL (R$)
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Homologated amount         R$ 892
Financial assets   R$ 16,279 R$ 16,866    
Remuneration payment period     8 years    
Extensions of concessions, tariff maximum amount   937 R$ 993    
Extension of the concessions   359 316    
Adjustment of BRR of transmission assets   149      
Net balance of assets deemed cost   R$ 816 816    
Grant acquisition period   30 years      
Pass-throughs of funds   R$ 794 586 R$ 341  
Financial assets [member]          
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Financial assets   R$ 4,917 7,084    
Hydro electric Plant [member]          
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Number of plants | Plant   18      
Cost of acquisition   R$ 2,216      
Other disposals of assets [member] | Regulated market [member]          
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Output sales percentage   70.00%      
Other disposals of assets [member] | Energy acquired in the Free Market [member]          
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Output sales percentage   30.00%      
Transmission Indemnity receivable [member] | Financial assets [member]          
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Financial assets   R$ 1,296 R$ 1,928    
Cemig GT [member]          
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Indemnity received R$ 1,139        
Contract assets [member]          
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]          
Transmission assets receivable   R$ 1,092