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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
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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

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