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Risk and Capital Management - Summary of Risks of Insurance and Private Pension (Detail) - ITAU UNIBANCO HOLDING [Member] - BRL (R$)
R$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Mandatory personal injury caused by motor vehicle [Member]      
Disclosure of insurance risk [Line Items]      
Insurance premiums   R$ 24 R$ 37
Retained premium   R$ 24 R$ 37
Retention   100.00% 100.00%
Extended warranty [Member]      
Disclosure of insurance risk [Line Items]      
Insurance premiums     R$ 112
Retained premium     R$ 112
Retention     100.00%
Group accident insurance [Member]      
Disclosure of insurance risk [Line Items]      
Insurance premiums R$ 690 R$ 667 R$ 780
Retained premium R$ 689 R$ 666 R$ 776
Retention 99.90% 99.80% 99.50%
Individual accident [Member]      
Disclosure of insurance risk [Line Items]      
Insurance premiums R$ 275 R$ 290 R$ 224
Retained premium R$ 280 R$ 289 R$ 212
Retention 101.80% 99.80% 94.80%
Home insurance in market policies credit life [Member]      
Disclosure of insurance risk [Line Items]      
Insurance premiums R$ 881 R$ 623 R$ 570
Retained premium R$ 879 R$ 621 R$ 570
Retention 99.80% 99.70% 100.00%
Group life [Member]      
Disclosure of insurance risk [Line Items]      
Insurance premiums R$ 934 R$ 1,001 R$ 1,278
Retained premium R$ 937 R$ 990 R$ 1,234
Retention 100.30% 98.90% 96.50%