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Financial Instruments and Risk Management - Summary of Risk of Increase in Inflation (Detail) - BRL (R$)
R$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial assets R$ 7,453 R$ 5,701
Interest rate risk [member]    
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial assets 4,376 4,187
Financial liabilities (4,805) (4,722)
Net assets (liabilities) exposed (429) (535)
Interest rate risk [member] | Loans, financings and debentures [member]    
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial liabilities (3,801) (3,935)
Interest rate risk [member] | Debt agreed with pension fund [Member]    
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial liabilities (721) (787)
Interest rate risk [member] | Forluz deficit of pension plan [member]    
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial liabilities (283)  
Interest rate risk [member] | Infrastructure -Distribution [Member]    
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial assets 111 128
Interest rate risk [member] | Transmission Indemnity receivable [member]    
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial assets 1,928 1,805
Interest rate risk [member] | Concession Grant Fee [Member]    
Disclosure of assets and liabilities with significant risk of material adjustment [line items]    
Financial assets R$ 2,337 R$ 2,254