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Stockholders' equity - Additional Information (Detail)
 12 Months Ended
Feb. 22, 2018
BRL (R$)
shares
Dec. 15, 2017
shares
Dec. 31, 2018
R$ / shares
shares
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Disclosure of classes of share capital [line items]          
Total share capital     9,804,135,348 9,825,771,656 9,873,461,313
Par value | R$ / shares     R$ 0    
Number of common stock     4,958,290,359    
Number of preferred stock     4,845,844,989    
Cancellation of Shares     14,424,206 47,689,657  
Percentage of stock split approved     0.50    
Annual minimum priority dividend | R$ / shares     R$ 0.015    
Bottom of range [member]          
Disclosure of classes of share capital [line items]          
Annual mandatory dividend rate     25.00%    
Preference shares [member]          
Disclosure of classes of share capital [line items]          
Total share capital     4,845,844,989 4,845,844,988 4,845,844,988
Annual minimum priority dividend | R$ / shares     R$ 0.022    
Ordinary shares [member]          
Disclosure of classes of share capital [line items]          
Total share capital     4,958,290,359 4,979,926,668 5,027,616,325
Cancellation of Shares 14,424,206 31,793,105 14,424,206 47,689,657  
Value of cancellation of shares considering effect of split | R$ R$ 47,689,657