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Financial Instruments and Risk Management - Schedule of Exposure to Exchange Rates (Detail)
R$ in Millions, $ in Millions
Dec. 31, 2017
BRL (R$)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
BRL (R$)
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) R$ 3,601   R$ 237
Base cenario [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 3,597 $ 3,601  
Base cenario [member] | Supply From Itaipu Binacional [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 240    
Base cenario [member] | Loans and Financings [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 3,357 4  
Probable Scenario [Member] | Three Point Four One One Brazil Real Per US Dollar [Member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 3,709    
Probable Scenario [Member] | Three Point Four One One Brazil Real Per US Dollar [Member] | Supply From Itaipu Binacional [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 248    
Probable Scenario [Member] | Three Point Four One One Brazil Real Per US Dollar [Member] | Loans and Financings [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 3,461    
Probable Scenario [Member] | Three Point Four One One Brazil Real Per US Dollar [Member] | Depreciation of twenty five percentage on brazil real [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) | $   3,713  
Net effect of exchange rate variation | $   112  
Probable Scenario [Member] | Three Point Four One One Brazil Real Per US Dollar [Member] | Depreciation of twenty five percentage on brazil real [member] | Loans and Financings [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) | $   4  
Possible Scenario [member] | Four Point Two Six Four Brazil Real Per US Dollar [Member] | Depreciation of twenty five percentage on brazil real [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 4,636 4,641  
Net effect of exchange rate variation | $   1,040  
Possible Scenario [member] | Four Point Two Six Four Brazil Real Per US Dollar [Member] | Depreciation of twenty five percentage on brazil real [member] | Supply From Itaipu Binacional [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 310    
Possible Scenario [member] | Four Point Two Six Four Brazil Real Per US Dollar [Member] | Depreciation of twenty five percentage on brazil real [member] | Loans and Financings [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 4,326 5  
Remote Scenario [member] | Five Point One One Seven Brazil Real Per US Dollar [Member] | Depreciation of fifty percentage on brazil real [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 5,563 5,570  
Net effect of exchange rate variation | $   1,969  
Remote Scenario [member] | Five Point One One Seven Brazil Real Per US Dollar [Member] | Depreciation of fifty percentage on brazil real [member] | Supply From Itaipu Binacional [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) 372    
Remote Scenario [member] | Five Point One One Seven Brazil Real Per US Dollar [Member] | Depreciation of fifty percentage on brazil real [member] | Loans and Financings [member]      
Disclosure of risk management strategy related to hedge accounting [line items]      
Loans and financings (Note 20) R$ 5,191 $ 7