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Concession Financial and Sector Assets and Liabilities - Summary of Generation Indemnity Receivable (Detail)
R$ in Millions
 12 Months Ended
Dec. 31, 2018
BRL (R$)
MW
Dec. 31, 2017
BRL (R$)
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Installed capacity (MW) | MW 3,601.70  
Net balance of assets based on historical cost R$ 204  
Net balance of assets based on fair value (replacement cost) 816 R$ 816
Monetary adjustment 322 317
Amounts received R$ (250) (234)
Usina Hidreletrica Tres marias [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 396.00  
Net balance of assets based on historical cost R$ 72  
Net balance of assets based on fair value (replacement cost) R$ 413  
Usina Hidreletrica Salto grande [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 102.00  
Net balance of assets based on historical cost R$ 11  
Net balance of assets based on fair value (replacement cost) R$ 39  
Usina Hidreletrica Itutinga [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 52.00  
Net balance of assets based on historical cost R$ 4  
Net balance of assets based on fair value (replacement cost) R$ 7  
Usina Hidreletrica Camargos [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 46.00  
Net balance of assets based on historical cost R$ 8  
Net balance of assets based on fair value (replacement cost) R$ 23  
Cemig PCH Piau [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 18.01  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) R$ 9  
Cemig PCH Gafanhoto [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 14.00  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) R$ 10  
Cemig PCH Peti [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 9.40  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) R$ 8  
Cemig PCH Dona Rita [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2013-09  
Installed capacity (MW) | MW 2.41  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) R$ 1  
Cemig PCH Tronqueiras [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 8.50  
Net balance of assets based on historical cost R$ 2  
Net balance of assets based on fair value (replacement cost) R$ 12  
Cemig PCH Joasal [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 8.40  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) R$ 8  
Cemig PCH Martins [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 7.70  
Net balance of assets based on historical cost R$ 2  
Net balance of assets based on fair value (replacement cost) R$ 4  
Cemig PCH Cajuru [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 7.20  
Net balance of assets based on historical cost R$ 4  
Net balance of assets based on fair value (replacement cost) R$ 4  
Cemig PCH Paciencia [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 4.08  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) R$ 4  
Cemig PCH Marmelos [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-07  
Installed capacity (MW) | MW 4.00  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) R$ 4  
Usina Hidreletrica Volta grande [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2017-02  
Installed capacity (MW) | MW 380.00  
Net balance of assets based on historical cost R$ 26  
Net balance of assets based on fair value (replacement cost) R$ 70  
Usina Hidreletrica Miranda [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2016-12  
Installed capacity (MW) | MW 408.00  
Net balance of assets based on historical cost R$ 27  
Net balance of assets based on fair value (replacement cost) R$ 23  
Usina Hidreletrica Jaguara [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2013-08  
Installed capacity (MW) | MW 424.00  
Net balance of assets based on historical cost R$ 40  
Net balance of assets based on fair value (replacement cost) R$ 174  
Usina Hidreletrica Sao simao [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Concession expiration date 2015-01  
Installed capacity (MW) | MW 1,710.00  
Net balance of assets based on historical cost R$ 1  
Net balance of assets based on fair value (replacement cost) 3  
Miranda and Sao Simao hydroelectric plants [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Net balance of assets based on historical cost   813
Adjustment regarding MME Order 291/17   215
Amounts based on MME Order 291/17   1,028
Monetary adjustment 55 56
Amounts received (1,139)  
Residual value of assets of Basic Plans   1,084
Miranda hydroelectric plant [member] | Miranda and Sao Simao hydroelectric plants [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Net balance of assets based on historical cost   610
Adjustment regarding MME Order 291/17   174
Amounts based on MME Order 291/17   784
Monetary adjustment 42 25
Amounts received (851)  
Residual value of assets of Basic Plans   809
Sao simao hydroelectric plant [member] | Miranda and Sao Simao hydroelectric plants [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [line items]    
Net balance of assets based on historical cost   203
Adjustment regarding MME Order 291/17   41
Amounts based on MME Order 291/17   244
Monetary adjustment 13 31
Amounts received R$ (288)  
Residual value of assets of Basic Plans   R$ 275