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Financial Instruments and Risk Management - Summary of Changes in Value of Options - The Difference Between Esimated Fair Value for Assets and Corresponding Exercise Price (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of financial liabilities [abstract]      
Balance R$ 1,150 R$ 1,245 R$ 166
Variation in fair value 187    
Variation in fair value   55 1,079
Written down, due to exercise of Put (830)    
Reversals   (150)  
Balance R$ 507 R$ 1,150 R$ 1,245