OBLIGATIONS WITH FIDC - INVESTMENT FUND IN CREDIT RIGHTS (Details) - BRL (R$) R$ in Thousands	Dec. 31, 2019	Dec. 31, 2018
OBLIGATIONS WITH FIDC - INVESTMENT FUND IN CREDIT RIGHTS
Obligations with FIDC	R$ 1,018,501	R$ 938,526

Noncurrent portion of amount payable regarding repurchase receivables from the Credit Right Investment Fund.

No definition available.

Name:	ggb_NoncurrentObligationsWithSecuritiesAndExchangeCommissionOfBrazil
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant