RELATED-PARTY TRANSACTIONS - Operations with related parties (Details) - BRL (R$) R$ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Controlling shareholders
RELATED PARTY TRANSACTIONS
Guarantee fee percentage	0.95%	0.95%	0.95%
Debenture guarantee fees	R$ 68	R$ 194	R$ 689
Revenue from rental agreement	554	445	801
Investments
RELATED PARTY TRANSACTIONS
Sales to Related parties	1,572,618	1,382,584	630,190
Purchases from related parties	198,636	129,513	121,618
Net amount, related party transactions	R$ 1,373,982	R$ 1,253,071	R$ 508,572

Reflects the guarantee fee as a percentage on the amount of debentures guaranteed.

No definition available.

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- Definition

The expense related to the guarantee of debentures.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The net revenue (purchases) from transactions with related parties.

+ References

No definition available.

+ Details

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

- Definition

Revenue from rental agreement entered with related party.

+ References

No definition available.

+ Details

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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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Data Type:	xbrli:stringItemType
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- Definition

The amount of goods purchased by the entity in related party transactions. [Refer: Related parties [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 24
-IssueDate 2019-01-01
-Paragraph 21
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=24&code=ifrs-tx-2019-en-r&anchor=para_21_a&doctype=Standard
-URIDate 2019-03-27

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

The amount of revenue arising from the sale of goods in related party transactions. [Refer: Revenue; Related parties [member]]

+ References

+ Details

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- Details

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- Details

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