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INSURANCE (Details) - BRL (R$) R$ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Equity |  |
Disclosure of types of insurance contracts [line items] |  |
Amount of assets or liabilities covered under insurance contracts | R$ 57,062,113 | R$ 60,924,807
Business Interruption |  |
Disclosure of types of insurance contracts [line items] |  |
Amount of assets or liabilities covered under insurance contracts | 6,464,512 | 8,354,147
Civil Liability |  |
Disclosure of types of insurance contracts [line items] |  |
Amount of assets or liabilities covered under insurance contracts | R$ 535,896 | R$ 527,974

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- Definition The amount of assets and/or liabilities covered under insurance contracts. + References No definition available. + Details
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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
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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