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Customers, Traders and Power Transport Concession Holders - Summary of Advances from Customers (Detail)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
BRL (R$)
Disclosure Of Advances From Customers [line items]  
Beginning Balance R$ 181
Ending Balance 233
Cemig GT [member]  
Disclosure Of Advances From Customers [line items]  
Beginning Balance 181
Addition 325
Supply completed (318)
Monetary adjustment 45
Ending Balance R$ 233