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RELATED-PARTY TRANSACTIONS - Operations with related parties, guarantees granted (Details) - Financing Agreements - BRL (R$)
R$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Joint-venture | Diaco S.A.      
Guarantees granted      
Original Amount R$ 137,700    
Carrying Amount   R$ 128,019  
Joint-venture | Gerdau Corsa S.A.P.I de C.V.      
Guarantees granted      
Original Amount 1,875,345    
Carrying Amount 1,933,929 1,797,856 R$ 2,016,260
Joint-venture | Gerdau Summit Aos Fundidos e Forjados S.A.      
Guarantees granted      
Original Amount 130,164    
Carrying Amount 41,571 6,550  
Subsidiary and Joint-venture | Gerdau Metaldom S.A; Aceros Corsa S.A De C.V      
Guarantees granted      
Carrying Amount R$ 198,619 R$ 178,200