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Financial Instruments and Risk Management - Schedule of Exposure to Exchange Rates (Detail) - BRL (R$)
R$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed R$ 6,150 R$ 3,601
Base cenario [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 6,150  
Base cenario [member] | Supply From Itaipu Binacional [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 268  
Base cenario [member] | Loans and Financings [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 5,882  
Probable scenario [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 6,032  
Probable scenario [member] | Supply From Itaipu Binacional [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 263  
Probable scenario [member] | 3.80 Brazil real per US dollar [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 6,032  
Net effect of exchange rate fluctuation (118)  
Probable scenario [member] | Loans and Financings [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 5,769  
Possible scenario [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 7,540  
Possible scenario [member] | Supply From Itaipu Binacional [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 328  
Possible scenario [member] | 4.75 Brazil real per US dollar [member] | Depreciation of 25% on Brazil real [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 7,540  
Net effect of exchange rate fluctuation 1,390  
Possible scenario [member] | Loans and Financings [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 7,212  
Remote scenario [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 9,048  
Remote scenario [member] | Supply From Itaipu Binacional [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 394  
Remote scenario [member] | 5.70 Brazil real per US dollar [member] | Depreciation of 50% on Brazil real [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed 9,048  
Net effect of exchange rate fluctuation 2,898  
Remote scenario [member] | Loans and Financings [member]    
Disclosure of risk management strategy related to hedge accounting [line items]    
Net liabilities exposed R$ 8,654