Name:	ggb_BusinessCombinationBargainPurchase
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Data Type:	xbrli:monetaryItemType
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The fair value, at acquisition date, of cash and cash equivalents transferred in a business combination.

No definition available.

Name:	ggb_CashAndCashEquivalentsAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

The fair value, at acquisition date, of current liabilities transferred in a business combination.

No definition available.

Name:	ggb_CurrentLiabilitiesAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
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The fair value, at acquisition date, of deferred income tax liabilities transferred in a business combination.

No definition available.

Name:	ggb_DeferredTaxLiabilitiesAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- References

No definition available.

Name:	ggb_DisclosureOfAcquisitionOfSubsidiaryAbstract
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The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

No definition available.

Name:	ggb_IfrsPaymentsToAcquireBusinessesNetOfCashAcquired
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Data Type:	xbrli:monetaryItemType
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The fair value, at acquisition date, of inventories transferred in a business combination.

No definition available.

Name:	ggb_InventoriesAcquisitionDateFairValue
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Data Type:	xbrli:monetaryItemType
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The fair value, at acquisition date, of net identifiable assets (liabilities) before adjustment for bargain purchase, transferred in a business combination.

No definition available.

Name:	ggb_NetIdentifiableAssetsLiabilitiesBeforeAdjustmentForBargainPurchaseAcquisitionDateFairValue
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Data Type:	xbrli:monetaryItemType
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The fair value, at acquisition date, of non-controlling interest transferred in a business combination.

No definition available.

Name:	ggb_NonControllingInterestAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
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The fair value, at acquisition date, of other current assets transferred in a business combination.

No definition available.

Name:	ggb_OtherCurrentAssetsAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

The fair value, at acquisition date, of other non-current assets transferred in a business combination.

No definition available.

Name:	ggb_OtherNonCurrentAssetsAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The fair value, at acquisition date, of other non-current liabilities transferred in a business combination.

No definition available.

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

Production capacity, in tons.

No definition available.

Name:	ggb_ProductionCapacityInTons
Namespace Prefix:	ggb_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	instant

The fair value, at acquisition date, of property, plant and equipment transferred in a business combination.

No definition available.

Name:	ggb_PropertyPlantAndEquipmentAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The fair value, at acquisition date, of trade accounts receivable, net, transferred in a business combination.

No definition available.

Name:	ggb_TradeAccountsReceivableNetAcquisitionDateFairValue
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The fair value, at acquisition date, of the consideration transferred in a business combination. [Refer: Business combinations [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 3
-IssueDate 2020-01-01
-Paragraph B64
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=3&code=ifrs-tx-2020-en-r&anchor=para_B64_f&doctype=Appendix&subtype=B
-URIDate 2020-01-01

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

- References

No definition available.

Name:	ifrs-full_AcquisitiondateFairValueOfTotalConsiderationTransferredAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
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The amount of cash and cash equivalents in subsidiaries or other businesses over which control is obtained or lost. [Refer: Subsidiaries [member]; Cash and cash equivalents]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2020-01-01
-Paragraph 40
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2020-en-r&anchor=para_40_c&doctype=Standard
-URIDate 2020-01-01

Name:	ifrs-full_CashAndCashEquivalentsInSubsidiaryOrBusinessesAcquiredOrDisposed2013
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amount of consideration paid or received in respect of both obtaining and losing control of subsidiaries or other businesses. [Refer: Subsidiaries [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2020-01-01
-Paragraph 40
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2020-en-r&anchor=para_40_a&doctype=Standard
-URIDate 2020-01-01

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

The percentage of voting equity interests acquired in a business combination. [Refer: Business combinations [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 3
-IssueDate 2020-01-01
-Paragraph B64
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=3&code=ifrs-tx-2020-en-r&anchor=para_B64_c&doctype=Appendix&subtype=B
-URIDate 2020-01-01