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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