Interest income (expense) on loans with related parties.

No definition available.

Name:	ggb_InterestIncomeExpenseOnLoansWithRelatedParties
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of non-current payables due to related parties. [Refer: Related parties [member]; Payables to related parties]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 78
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_78&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_NoncurrentPayablesToRelatedParties
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of non-current receivables due from related parties. [Refer: Related parties [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 78
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_78_b&doctype=Standard
-URIDate 2018-03-16

+ Details

Name:	ifrs-full_NoncurrentReceivablesDueFromRelatedParties
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfRelatedPartiesAxis=ggb_GerdauCorsaSapiDeCvMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfRelatedPartiesAxis=ggb_FundacaoGerdauMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

RELATED-PARTY TRANSACTIONS - Intercompany loans (Details) - BRL (R$) R$ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
RELATED PARTY TRANSACTIONS
Assets	R$ 27,939	R$ 51,839
Liabilities	(1,350)
Net financial income	545	95	R$ (2,457)
Diaco S.A.
RELATED PARTY TRANSACTIONS
Liabilities	(1,350)
Gerdau Corsa S.A.P.I de C.V.
RELATED PARTY TRANSACTIONS
Assets	72	7
Fundacao Gerdau
RELATED PARTY TRANSACTIONS
Assets	R$ 27,867	R$ 51,832