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Operating Context - Additional Information (Detail)
R$ in Millions, $ in Billions
 12 Months Ended
Dec. 31, 2017
BRL (R$)
Plant
MW
Dec. 31, 2017
USD ($)
Plant
MW
Dec. 31, 2016
BRL (R$)
Dec. 31, 2015
BRL (R$)
Disclosure of operating context [line items]        
Current liabilities exceeded current assets amount | R$ R$ (126)      
Short-term loans, financings and debentures | R$ 2,371   R$ 4,837  
Long-term loans, financings and debentures | R$ 12,027   10,342  
Positive operating cash flows | R$ 580   1,213 R$ 3,008
Indebtedness from loans, financings and debentures | R$ 14,398   R$ 15,179  
Reprofiling of debt | R$ R$ 3,400      
Baguari Energia [member]        
Disclosure of operating context [line items]        
Ownership interest 49.00% 49.00%    
Neoenergia [member]        
Disclosure of operating context [line items]        
Ownership interest 51.00% 51.00%    
Alianca Norte [member] | Vale [member]        
Disclosure of operating context [line items]        
Ownership interest 50.10% 50.10%    
Alianca Norte [member] | Norte Energia [member]        
Disclosure of operating context [line items]        
Interest acquisition percentage 9.00%      
Amazonia energia participacoes S A [member] | Light SA [member]        
Disclosure of operating context [line items]        
Ownership interest 25.50% 25.50%    
Amazonia energia participacoes S A [member] | Norte Energia S.A [Member]        
Disclosure of operating context [line items]        
Ownership interest 9.77% 9.77%    
Alianca [member]        
Disclosure of operating context [line items]        
Plant capacity 1,158 1,158    
Assured energy offtake level of plant 652 652    
Alianca [member] | Vale [member]        
Disclosure of operating context [line items]        
Percentage of total capital holding 55.00% 55.00%    
Transmissora Alianca de Energia Eletrica S.A. [member] | Brazil [member]        
Disclosure of operating context [line items]        
Principal place of business of joint operation 17 states 17 states    
Axxiom solucoesTecnologicas S.A. [member]        
Disclosure of operating context [line items]        
Ownership interest 49.00% 49.00%    
Axxiom solucoesTecnologicas S.A. [member] | Light [member]        
Disclosure of operating context [line items]        
Ownership interest 51.00% 51.00%    
Cemig GT [member]        
Disclosure of operating context [line items]        
Debt | $   $ 1    
Number of power plants | Plant 60 60    
Cemig GT [member] | Alianca Norte [member]        
Disclosure of operating context [line items]        
Ownership interest 49.90% 49.90%    
Cemig GT [member] | Alianca [member]        
Disclosure of operating context [line items]        
Percentage of total capital holding 45.00% 45.00%    
Cemig GT [member] | Hydroelectrical power plants [member]        
Disclosure of operating context [line items]        
Number of power plants | Plant 56 56    
Cemig GT [member] | Wind power plants [member]        
Disclosure of operating context [line items]        
Number of power plants | Plant 3 3    
Cemig GT [member] | Thermal power plant [member]        
Disclosure of operating context [line items]        
Number of power plants | Plant 1 1    
Cemig Gerao Trs Marias S.A. [member]        
Disclosure of operating context [line items]        
Plant capacity 396 396    
Assured energy offtake level of plant 239 239    
Cemig Gerao Salto Grande S.A. [member]        
Disclosure of operating context [line items]        
Plant capacity 102 102    
Assured energy offtake level of plant 75 75    
Cemig Gerao Camargos S.A. [member]        
Disclosure of operating context [line items]        
Plant capacity 46 46    
Assured energy offtake level of plant 21 21    
Cemig Geracao Itutinga S.A. [member]        
Disclosure of operating context [line items]        
Plant capacity 28 28    
Assured energy offtake level of plant 52 52    
Cemig Geracao Lest S.A. [member]        
Disclosure of operating context [line items]        
Plant capacity 35.16 35.16    
Assured energy offtake level of plant 18.64 18.64    
Cemig Geracao Oeste S.A. [member]        
Disclosure of operating context [line items]        
Plant capacity 28.90 28.90    
Assured energy offtake level of plant 11.21 11.21    
Cemig Geracao Sul S.A. [member]        
Disclosure of operating context [line items]        
Plant capacity 39.53 39.53    
Assured energy offtake level of plant 27.42 27.42    
Cemig Telecomunicacoes S.A. [member] | Ativas Data Center [member]        
Disclosure of operating context [line items]        
Ownership interest 19.60% 19.60%    
Ceming GT [Member] | Amazonia energia participacoes S A [member]        
Disclosure of operating context [line items]        
Ownership interest 74.50% 74.50%