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Financial Instruments and Risk Management - Summary of Derivative Instruments Contracted (Detail)
R$ in Millions, $ in Millions
 1 Months Ended 12 Months Ended
Jul. 31, 2018
Dec. 31, 2018
BRL (R$)
Dec. 31, 2017
BRL (R$)
Dec. 31, 2018
USD ($)
Derivative Financial Instruments [line items]        
Cemig's obligation 125.52% of the CDI rate   150.49% of the CDI  
Unrealized gain/loss Carrying amount   R$ 712 R$ 51  
Unrealized gain/loss fair value   R$ 813 (32)  
49% of Certificados de Deposito Inter bancario [member]        
Derivative Financial Instruments [line items]        
Cemig's right   US$ exchange variation + Rate (9,25% p.y.)    
Cemig's obligation   Local currency + R$ 150.49% of CDI    
Maturity period   Interest: Half-yearly Principal: Dec. 2024    
Notional amount | $       $ 1,000
Unrealized gain/loss Carrying amount   R$ 679 51  
Unrealized gain/loss fair value   R$ 627 R$ (32)  
52% of Certificados de Deposito Inter bancario [member]        
Derivative Financial Instruments [line items]        
Cemig's right   US$ exchange variation + Rate (9,25% a.a.)    
Cemig's obligation   Local currency + R$125.52% of CDI    
Maturity period   Interest: Half-yearly Principal: Dec. 2024    
Notional amount | $       $ 500
Unrealized gain/loss Carrying amount   R$ 33    
Unrealized gain/loss fair value   R$ 186