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Concession Financial Assets and Liabilities - Summary of Generation Indemnity Receivable (Detail)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
BRL (R$)
MW
Dec. 31, 2016
BRL (R$)
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$ 547
Monitory adjustment (C) 317 R$ 352
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 814  
Net balance of assets based on fair value (replacement cost) 838  
Residual value of assets of basis project based on replacement of cost at December 2017 813  
Adjustment 215  
Amounts based on MME Order 291 (A)+(B) 1,028  
Monitory adjustment (C) 56  
Residual value of assets of basis project at 2017 R$ 1,084  
Miranda and Sao Simao hydroelectric plants [member] | Miranda hydroelectric plant [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [Line Items]    
Concession expiration date 2016-12  
Net balance of assets based on historical cost R$ 751  
Net balance of assets based on fair value (replacement cost) 632  
Residual value of assets of basis project based on replacement of cost at December 2017 610  
Adjustment 174  
Amounts based on MME Order 291 (A)+(B) 784  
Monitory adjustment (C) 25  
Residual value of assets of basis project at 2017 R$ 809  
Miranda and Sao Simao hydroelectric plants [member] | Sao simao hydroelectric plant [member]    
Disclosure Of Financial Assets and Liabilities Of The Concession [Line Items]    
Concession expiration date 2015-01  
Net balance of assets based on historical cost R$ 63  
Net balance of assets based on fair value (replacement cost) 206  
Residual value of assets of basis project based on replacement of cost at December 2017 203  
Adjustment 41  
Amounts based on MME Order 291 (A)+(B) 244  
Monitory adjustment (C) 31  
Residual value of assets of basis project at 2017 R$ 275  
Tres Marias Hydroelectric Plant [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$ 71  
Net balance of assets based on fair value (replacement cost) R$ 413  
Salto Grande Hydroelectric Plant [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  
Itutinga Hydroelectric Plant [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  
Camargos hydroelectric plant [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  
Piau Small Hydroelectric Plant [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$ 2  
Net balance of assets based on fair value (replacement cost) R$ 9  
Gafanhoto Small Hydroelectric Plant [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  
Peti Small Hydroelectric Plant [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  
Dona Rita Small Hydroelectric Plant [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  
Tronqueiras Small Hydroelectric Plant [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  
Joasal Small Hydroelectric Plant [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  
Martins Small Hydroelectric Plant [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  
Cajuru Small Hydroelectric Plant [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  
Paciencia Small Hydroelectric Plant [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  
Marmelos Small Hydroelectric Plant [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  
Volta grande hydroelectric plant [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  
Miranda hydroelectric plant [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$ 26  
Net balance of assets based on fair value (replacement cost) R$ 23  
Jaguara hydroelectric plant [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  
Sao simao hydroelectric plant [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$ 2  
Net balance of assets based on fair value (replacement cost) R$ 3