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Equity and Remuneration to Shareholders - Additional Information (Detail) |  |  |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Apr. 30, 2018 BRL (R$) | Apr. 23, 2018 BRL (R$) R$ / shares shares | Oct. 26, 2017 BRL (R$) R$ / shares shares | Apr. 30, 2018 BRL (R$) | Dec. 31, 2017 BRL (R$) Installments R$ / shares shares | Dec. 31, 2016 BRL (R$) R$ / shares shares | Dec. 31, 2015 BRL (R$) R$ / shares shares | Mar. 31, 2018 BRL (R$)
Disclosure of reserves within equity [line items] |  |  |  |  |  |  |  |
Share capital |  |  |  |  | R$ 6,294,000,000 | R$ 6,294,000,000 | R$ 6,294,000,000 |
Shares issued | shares |  |  |  |  | 1,258,841,654 | 1,258,841,654 | 1,258,841,654 |
Par value per share | R$ / shares |  |  |  |  | R$ 5.00 | R$ 5.00 | R$ 5.00 |
Percentage of right to reduction in income tax |  |  |  |  | 75.00% |  |  |
Period of right to reduction in income tax |  |  |  |  | 10 years |  |  |
Right to reduction in income tax, starting period |  |  |  |  | 2014 |  |  |
Incentives tax |  |  |  |  | R$ 1,000,000 | R$ 7,000,000 | R$ 21,000,000 |
Incentives tax reserve |  |  |  |  | R$ 58,000,000 | 57,000,000 | 50,000,000 |
Mandatory dividends payout percentage |  |  |  |  | 50.00% |  |  |
Minimum annual dividend percentage on par value |  |  |  |  | 6.00% |  |  |
Number of installments for dividend paid | Installments |  |  |  |  | 2 |  |  |
Retained earnings |  |  |  |  | R$ 5,729,000,000 | R$ 5,200,000,000 | R$ 4,663,000,000 |
Capital increase |  | R$ 1,000,000,000 | R$ 1,000,000,000 |  |  |  |  |
New shares authorized | shares |  | 199,910,947 | 199,910,947 |  |  |  |  |
Par value per share | R$ / shares |  | R$ 5.00 | R$ 5.00 |  |  |  |  |
Percentage of equity interest held for new shares |  |  | 15.89% |  |  |  |  |
Shares subscribed |  |  |  |  | R$ 1,215,000,000 |  |  |
Number of shares not subscribed | shares |  |  |  |  | 14,945,429 |  |  |
Major ordinary share transactions [member] |  |  |  |  |  |  |  |
Disclosure of reserves within equity [line items] |  |  |  |  |  |  |  |
Share capital |  |  |  |  |  |  |  | R$ 7,294,000,000
Proposed divided | R$ 1,001,000,000 |  |  |  |  |  |  |
Retained earnings | 28,000,000 |  |  | R$ 28,000,000 |  |  |  |
Equity in retained earnings reserve | 528,000,000 |  |  |  |  |  |  |
Equity in incentives tax reserve | R$ 1,000,000 |  |  |  |  |  |  |
Common shares [member] |  |  |  |  |  |  |  |
Disclosure of reserves within equity [line items] |  |  |  |  |  |  |  |
Shares issued | shares |  |  |  |  | 420,764,708 | 420,764,708 | 420,764,708 |
New shares authorized | shares |  | 66,849,505 | 66,849,505 |  |  |  |  |
Par value per share | R$ / shares |  |  | R$ 6.57 |  |  |  |  |
Number of shares not subscribed | shares |  |  |  |  | 13,139,679 |  |  |
Common shares [member] | Major ordinary share transactions [member] |  |  |  |  |  |  |  |
Disclosure of reserves within equity [line items] |  |  |  |  |  |  |  |
Minimum mandatory dividend payable |  |  |  | 14,000,000 |  |  |  |
Preference shares [member] |  |  |  |  |  |  |  |
Disclosure of reserves within equity [line items] |  |  |  |  |  |  |  |
Shares issued | shares |  |  |  |  | 838,076,946 | 838,076,946 | 838,076,946 |
Minimum annual dividend percentage on par value |  |  |  |  | 10.00% |  |  |
Minimum annual dividend percentage on equity portion |  |  |  |  | 3.00% |  |  |
Minimum mandatory dividend payable |  |  |  |  | R$ 486,000,000 |  |  |
New shares authorized | shares |  | 133,061,442 | 133,061,442 |  |  |  |  |
Par value per share | R$ / shares |  |  | R$ 6.57 |  |  |  |  |
Number of shares not subscribed | shares |  |  |  |  | 1,815,750 |  |  |
Preference shares [member] | Major ordinary share transactions [member] |  |  |  |  |  |  |  |
Disclosure of reserves within equity [line items] |  |  |  |  |  |  |  |
Minimum mandatory dividend payable |  |  |  | R$ 486,000,000 |  |  |  |

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