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Financial Instruments and Risk Management - Summary of Exposure to Exchange Rates (Detail)
€ in Millions, R$ in Millions, $ in Millions
Dec. 31, 2017
BRL (R$)
Dec. 31, 2017
EUR (€)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
BRL (R$)
Dec. 31, 2016
EUR (€)
Dec. 31, 2016
USD ($)
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed R$ 3,601     R$ 237    
US dollar [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed 3,597     230    
US dollar [member] | Supply From Itaipu Binacional [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed 240     207    
Foreign currency risk [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed | $     $ 1,089     $ 69
Foreign currency risk [member] | Supply From Itaipu Binacional [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed | $     74     62
Loans and Financings [member] | Foreign currency risk [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed | €   € 1     € 2  
Loans and Financings [member] | US dollar [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed 3,357     23    
Loans and Financings [member] | Euro [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed R$ 4     R$ 7    
Loans and Financings [member] | Foreign currency risk [member]            
Disclosure of risk management strategy related to hedge accounting [line items]            
Net liabilities exposed | $     $ 1,015     $ 7