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Related Party Transactions - Summary of Financial Investments in Securities of Related Parties, in the Investment Fund (Parenthetical) (Detail) - Related parties [member]
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Investment [Line Items]    
Percentage of investment 72.28% 78.31%
Cemig [member]    
Investment [Line Items]    
Percentage of investment 4.17% 10.12%
Cemig GT [member]    
Investment [Line Items]    
Percentage of investment 26.85% 20.86%
Cemig D [member]    
Investment [Line Items]    
Percentage of investment 19.90% 24.94%
Other Subsidiaries [Member]    
Investment [Line Items]    
Percentage of investment 21.36% 22.39%