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Stockholders' equity - Summary of Cost of Shares Purchased and Average Cost of Treasury Shares and Market Price (Detail) - R$ / shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Ordinary shares [member]    
Disclosure of classes of share capital [line items]    
Average cost   R$ 24.70
Market price per share R$ 30.05 25.12
Ordinary shares [member] | Bottom of range [member]    
Disclosure of classes of share capital [line items]    
Cost per share   24.70
Ordinary shares [member] | Weighted average [member]    
Disclosure of classes of share capital [line items]    
Cost per share   24.70
Ordinary shares [member] | Top of range [member]    
Disclosure of classes of share capital [line items]    
Cost per share   24.70
Preference shares [member]    
Disclosure of classes of share capital [line items]    
Average cost 21.76 20.60
Market price per share 35.50 28.38
Preference shares [member] | Bottom of range [member]    
Disclosure of classes of share capital [line items]    
Cost per share 37.45 22.32
Preference shares [member] | Weighted average [member]    
Disclosure of classes of share capital [line items]    
Cost per share 38.95 24.16
Preference shares [member] | Top of range [member]    
Disclosure of classes of share capital [line items]    
Cost per share R$ 40.06 R$ 25.70