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Concessions and Authorizations - Additional Information (Detail)
R$ in Millions
 12 Months Ended
Sep. 27, 2017
BRL (R$)
MW
Aug. 03, 2017
BRL (R$)
Dec. 26, 2014
Dec. 31, 2017
BRL (R$)
Dec. 31, 2016
BRL (R$)
Dec. 31, 2015
BRL (R$)
Dec. 31, 2014
BRL (R$)
Disclosure of electricity concessions and authorizations [Line Items]              
Investment       R$ 618 R$ 801 R$ 861 R$ 906
Small hydro plants enterprise [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Discount of future payments       12.50%      
Conventional hydroelectric plants enterprise [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Discount of future payments       5.10%      
2019 [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Percentage of Selic rate       80.00%      
2020 [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Percentage of Selic rate       111.00%      
Second Amendment to Concession Contract [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Concession contract for electricity generation, additional period     30 years        
Original expiration date of concession contract     Jan. 10, 2023        
Extended expiration date of concession contract     Jan. 10, 2053        
Cemig Distribution [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Concession expired       Dec. 31, 2045      
Cemig GT [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Annual generation revenue of plants       R$ 462 319    
Residual value of the infrastructure assets   R$ 1,028   1,084      
Income from residual value of the infrastructure assets   271          
Investment       126 R$ 127    
Cemig GT [member] | Sao Simao Jaguara and Miranda hydroelectric plants [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Plant capacity | MW 2,922            
Sale price of plant R$ 12,131            
Cemig GT [member] | Sao simao hydroelectric plant [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Residual value of the infrastructure assets   244          
Investment       3      
Cemig GT [member] | Miranda hydroelectric plant [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Residual value of the infrastructure assets   R$ 784          
Investment       23      
Cemig GT [member] | Jaguara hydroelectric plant [member]              
Disclosure of electricity concessions and authorizations [Line Items]              
Investment       R$ 174