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Risks and Financial Instruments - Contractual undiscounted cash outflows (Parenthetical) (Detail)
 12 Months Ended
Dec. 31, 2018
2018 [member]  
Summary of risks and financial instruments [line items]  
CDI rate 6.39%
Exchange rate of the Real against the U.S. dollar 3.87
TJLP rate 7.03%
IGP-M rate 8.12%
IPCA rate 4.01%
From 2019 To 2021 [member]  
Summary of risks and financial instruments [line items]  
CDI rate 7.38%
From 2022 To 2023 [member]  
Summary of risks and financial instruments [line items]  
CDI rate 8.52%
From 2024 To 2033 [member]  
Summary of risks and financial instruments [line items]  
CDI rate 9.49%
2019 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 3.92
IGP-M rate 4.14%
2020 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 4.06
2021 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 4.27
2022 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 4.53
2023 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 4.81
2024 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 5.12
2025 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 5.44
2026 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 5.77
2027 [member]  
Summary of risks and financial instruments [line items]  
Exchange rate of the Real against the U.S. dollar 6.13
From 2020 To 2033 [member]  
Summary of risks and financial instruments [line items]  
IGP-M rate 3.90%