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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 |

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