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Financial Assets Held for Trading and Designated at Fair Value Through Profit or Loss - Summary of Financial Assets Designated at Fair Value Through Profit or Loss (Detail) - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2017
Disclosure of financial assets designated as measured at fair value through profit or loss [Line Items]      
Fair value R$ 286,646 R$ 273,536 R$ 218,805
Financial assets at fair value through profit or loss, designated upon initial recognition or subsequently, category [member]      
Disclosure of financial assets designated as measured at fair value through profit or loss [Line Items]      
Gross carrying amount 1,232 1,670  
Accumulated gain / (loss) reflected in income 37 76  
Fair value 1,269 1,746  
Financial assets at fair value through profit or loss, designated upon initial recognition or subsequently, category [member] | Brazilian external debt bonds [Member]      
Disclosure of financial assets designated as measured at fair value through profit or loss [Line Items]      
Gross carrying amount 1,232 1,670  
Accumulated gain / (loss) reflected in income 37 76  
Fair value R$ 1,269 R$ 1,746