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Financial Instruments and Risk Management - Summary of Derivative Instruments Contracted (Detail)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
BRL (R$)
Dec. 31, 2017
USD ($)
Derivative financial instruments [Abstract]    
emig's right US$: FX variation + ate (9.25% p.a.) US$: FX variation + ate (9.25% p.a.)
Cemig's obligatio R$: 150.49% of CDI R$: 150.49% of CDI
n Maturity period 12/2017 to 12/2024 12/2017 to 12/2024
principal racted | $   $ 1,000,000
Unrealized ga Amount according to contract 2017 R$ 51  
in /loss Fair value 2017 R$ (32)