RELATED PARTY TRANSACTIONS - Operations with related parties (Details) - BRL (R$) R$ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Controlling shareholders
RELATED PARTY TRANSACTIONS
Guarantee fee percentage	0.95%	0.95%
Debenture guarantee fees	R$ 689	R$ 4,732
Revenue from rental agreement	801	1,001
Investments
RELATED PARTY TRANSACTIONS
Sales to Related parties	630,190	421,415
Purchases from related parties	121,618	141,275
Net amount, related party transactions	R$ 508,572	R$ 280,140

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

No definition available.

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Namespace Prefix:	ggb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The expense related to the guarantee of debentures.

+ References

No definition available.

+ Details

Name:	ggb_DebentureGuaranteeFees
Namespace Prefix:	ggb_
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

Name:	ggb_NetRevenuePurchasesFromTransactionsWithRelatedParties
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

Name:	ggb_RevenueFromRentalAgreementWithRelatedParty
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

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

- 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 2017-01-01
-Paragraph 21
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2017-03-01&anchor=para_21_a&doctype=Standard
-URIDate 2017-03-09

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
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

Name:	ifrs-full_RevenueFromSaleOfGoodsRelatedPartyTransactions
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

Name:	ifrs-full_ClassesOfAssetsAxis=ifrs-full_InvestmentsAccountedForUsingEquityMethodMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: