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RELATED-PARTY TRANSACTIONS - Guarantees granted (Details) - Joint-venture - Financial guarantees - BRL (R$)
R$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Gerdau Corsa S.A.P.I de C.V.      
Guarantees granted      
Original Amount R$ 4,905,367    
Carrying Amount 2,157,290 R$ 2,242,865 R$ 1,784,868
Gerdau Summit Acos Fundidos e Forjados S.A.      
Guarantees granted      
Original Amount 130,164    
Carrying Amount   R$ 30,125 37,285
Gerdau Metaldom S.A; Aceros Corsa S.A De C.V      
Guarantees granted      
Original Amount R$ 274,560    
Carrying Amount     R$ 241,842