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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

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