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Securities Sold Under Repurchase Agreements and Interbank and Institutional Market Debts - Summary of Securities Sold Under Repurchase Agreements (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2017
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements R$ 330,237 R$ 312,634 R$ 349,164
Assets Pledged as Collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 77,651 109,253  
Assets Pledged as Collateral [member] | Government securities - abroad [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 46,680 43,491  
Assets Pledged as Collateral [member] | Corporate securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 9,051 6,564  
Assets Pledged as Collateral [member] | Own issue [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 21,417 58,837  
Assets Pledged as Collateral [member] | Foreign securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements R$ 503 361  
Assets Pledged as Collateral [member] | Bottom of range [member] | Government securities - abroad [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 93.50%    
Assets Pledged as Collateral [member] | Bottom of range [member] | Corporate securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 40.00%    
Assets Pledged as Collateral [member] | Bottom of range [member] | Own issue [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 65.00%    
Assets Pledged as Collateral [member] | Bottom of range [member] | Foreign securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 0.22%    
Assets Pledged as Collateral [member] | Top of range [member] | Government securities - abroad [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 6.40%    
Assets Pledged as Collateral [member] | Top of range [member] | Corporate securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 97.70%    
Assets Pledged as Collateral [member] | Top of range [member] | Own issue [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 16.93%    
Assets Pledged as Collateral [member] | Top of range [member] | Foreign securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 5.20%    
Assets Received as Collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements R$ 172,953 146,853  
Assets Received as Collateral [member] | Bottom of range [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 5.60%    
Assets Received as Collateral [member] | Top of range [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 6.40%    
Right to sell or repledge collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements R$ 79,633 56,528  
Right to sell or repledge collateral [member] | Bottom of range [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 97.50%    
Right to sell or repledge collateral [member] | Top of range [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Percentage of securities sold under repurchase agreements 10.00%    
Current [Member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements R$ 271,521 240,808  
Current [Member] | Assets Pledged as Collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 71,231 82,075  
Current [Member] | Assets Pledged as Collateral [member] | Government securities - abroad [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 46,676 43,491  
Current [Member] | Assets Pledged as Collateral [member] | Corporate securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 9,051 6,564  
Current [Member] | Assets Pledged as Collateral [member] | Own issue [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 15,156 31,659  
Current [Member] | Assets Pledged as Collateral [member] | Foreign securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 348 361  
Current [Member] | Assets Received as Collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 172,953 146,853  
Current [Member] | Right to sell or repledge collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 27,337 11,880  
Non-current [Member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 58,716 71,826  
Non-current [Member] | Assets Pledged as Collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 6,420 27,178  
Non-current [Member] | Assets Pledged as Collateral [member] | Government securities - abroad [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 4    
Non-current [Member] | Assets Pledged as Collateral [member] | Own issue [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 6,261 27,178  
Non-current [Member] | Assets Pledged as Collateral [member] | Foreign securities [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements 155    
Non-current [Member] | Right to sell or repledge collateral [member]      
Disclosure Of Repurchase Agreements [Line Items]      
Securities sold under repurchase agreements R$ 52,296 R$ 44,648