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Intangible Assets - Additional Information (Detail) - BRL (R$)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of detailed information about intangible assets [line items]      
Additions of intangible asset R$ 1,143,000,000    
Financing costs on Intangible assets 71,000,000 R$ 142,000,000 R$ 159,000,000
Intangible assets with non-defined useful life R$ 0    
Annual average amortization rate 3.85 percentage    
Cemig D [member]      
Disclosure of detailed information about intangible assets [line items]      
Intangible assets R$ 2,368,000,000 R$ 2,243,000,000