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INSURANCE (Details) - BRL (R$) R$ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Equity
INSURANCE
Amount of assets or liabilities covered under insurance contracts	R$ 64,676,890	R$ 57,062,113
Business Interruption
INSURANCE
Amount of assets or liabilities covered under insurance contracts	6,610,851	6,464,512
Civil Liability
INSURANCE
Amount of assets or liabilities covered under insurance contracts	R$ 627,718	R$ 535,896

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- Definition

The amount of assets and/or liabilities covered under insurance contracts.

+ References

No definition available.

+ Details

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

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

X

- Details

Name:	ifrs-full_TypesOfInsuranceContractsAxis=ggb_EquityInsuranceContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_TypesOfInsuranceContractsAxis=ggb_BusinessInterruptionInsuranceContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_TypesOfInsuranceContractsAxis=ggb_CivilLiabilityInsuranceContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: