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Operating Context - Additional Information (Detail)
R$ in Millions
 12 Months Ended
Dec. 31, 2018
BRL (R$)
Plant
MW
Dec. 31, 2017
MW
Disclosure of operating context [line items]    
Percentage of aggregate voting stock due to disposal 49.99%  
Put options [member]    
Disclosure of operating context [line items]    
Commitments settled to share holders in relation to put option | R$ R$ 659  
Baguari Energia [member]    
Disclosure of operating context [line items]    
Ownership interest 49.00%  
Neoenergia [member]    
Disclosure of operating context [line items]    
Ownership interest 51.00%  
Alianca [member]    
Disclosure of operating context [line items]    
Plant capacity   1,170
Assured energy offtake level of plant   668
BRAZIL | Transmissora Alianca de Energia Eletrica S.A. [member]    
Disclosure of operating context [line items]    
Principal place of business of joint operation 17 states  
Light [member] | Axxiom solucoesTecnologicas S.A. [member]    
Disclosure of operating context [line items]    
Percentage of total capital holding 51.00%  
Norte Energia [member] | Alianca Norte [member]    
Disclosure of operating context [line items]    
Interest acquisition percentage 9.00%  
Cemig GT [member] | Axxiom solucoesTecnologicas S.A. [member]    
Disclosure of operating context [line items]    
Percentage of total capital holding 49.00%  
Vale [member] | Alianca Norte [member]    
Disclosure of operating context [line items]    
Ownership interest 50.10%  
Vale [member] | Alianca [member]    
Disclosure of operating context [line items]    
Percentage of total capital holding   55.00%
Cemig GT [member]    
Disclosure of operating context [line items]    
Number of power plants | Plant 64  
Plant capacity 3,509  
Cemig GT [member] | Hydroelectrical power plants [member]    
Disclosure of operating context [line items]    
Number of power plants | Plant 60  
Cemig GT [member] | Wind power projects [member]    
Disclosure of operating context [line items]    
Number of power plants | Plant 2  
Cemig GT [member] | Thermal power plant [member]    
Disclosure of operating context [line items]    
Number of power plants | Plant 1  
Cemig GT [member] | Solar power plant [member]    
Disclosure of operating context [line items]    
Number of power plants | Plant 1  
Cemig GT [member] | Alianca Norte [member]    
Disclosure of operating context [line items]    
Ownership interest 49.90%  
Cemig GT [member] | Alianca [member]    
Disclosure of operating context [line items]    
Percentage of total capital holding   45.00%
Amazonia energia participacoes S A [member]    
Disclosure of operating context [line items]    
Ownership interest 74.50%  
Amazonia energia participacoes S A [member] | Light [member]    
Disclosure of operating context [line items]    
Ownership interest 25.50%  
Amazonia energia participacoes S A [member] | Norte Energia [member]    
Disclosure of operating context [line items]    
Interest acquisition percentage 9.77%  
Cemig Geracao Tres Marias SA [member]    
Disclosure of operating context [line items]    
Plant capacity 396  
Assured energy offtake level of plant 239  
Cemig Gerao Salto Grande S.A. [member]    
Disclosure of operating context [line items]    
Plant capacity 102  
Assured energy offtake level of plant 75  
Cemig Geracao Itutinga S.A. [member]    
Disclosure of operating context [line items]    
Plant capacity 52  
Assured energy offtake level of plant 28  
Cemig Gerao Camargos S.A. [member]    
Disclosure of operating context [line items]    
Plant capacity 46  
Assured energy offtake level of plant 21  
Cemig Geracao Sul S.A. [member]    
Disclosure of operating context [line items]    
Plant capacity 39.53  
Assured energy offtake level of plant 27.42  
Cemig Geracao Leste SA [member]    
Disclosure of operating context [line items]    
Plant capacity 35.16  
Assured energy offtake level of plant 18.64  
Cemig Geracao Oeste S.A. [member]    
Disclosure of operating context [line items]    
Plant capacity 28.90  
Assured energy offtake level of plant 11.21  
Fip Melbourne [member]    
Disclosure of operating context [line items]    
Percentage of investments held for share capital 83.00%