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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, 2019
Dec. 31, 2018
Dec. 31, 2017
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 1,750,249    
Carrying Amount 1,784,868 R$ 1,933,929 1,797,856
Joint-venture | Gerdau Summit Aos Fundidos e Forjados S.A.      
Guarantees granted      
Original Amount 130,164    
Carrying Amount 37,285 41,571 6,550
Subsidiary and Joint-venture | Gerdau Metaldom S.A; Aceros Corsa S.A De C.V      
Guarantees granted      
Original Amount 274,560    
Carrying Amount R$ 241,842 R$ 198,619 R$ 178,200