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EMPLOYEE BENEFITS - Defined benefit pension plan - Allocation of plan assets (Details) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Brazil |  |
Allocation for plan assets |  |
Fixed income | 98.00% | 98.00%
Others | 2.00% | 2.00%
Total | 100.00% | 100.00%
North America |  |
Allocation for plan assets |  |
Fixed income | 58.10% | 48.30%
Variable income | 36.50% | 45.00%
Others | 5.40% | 6.70%
Total | 100.00% | 100.00%
Defined benefit pension plan |  |
Allocation for plan assets |  |
Target allocation of shares (as a percent) | 50.00% | 50.00%
Target allocation of debt securities (as a percent) | 40.00% | 40.00%
Target allocation of alternative securities (as a percent) | 10.00% | 10.00%

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- Definition Target allocation range in alternative securities, in a defined benefit plan, represented as a percentage. + References No definition available. + Details
Name: | ggb_AllocationForPlanAllocationRangeForAlternativeSecuritiesPercentage
Namespace Prefix: | ggb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Target allocation range in debt securities, in a defined benefit plan, represented as a percentage. + References No definition available. + Details
Name: | ggb_AllocationForPlanAllocationRangeForDebtSecuritiesPercentage
Namespace Prefix: | ggb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Target allocation range in shares, in a defined benefit plan, represented as a percentage. + References No definition available. + Details
Name: | ggb_AllocationForPlanAllocationRangeForSharesPercentage
Namespace Prefix: | ggb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | ggb_AllocationForPlanAssetsAbstract
Namespace Prefix: | ggb_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Allocation of plan assets, in a defined benefit plan, represented by fixed income percentage. + References No definition available. + Details
Name: | ggb_AllocationForPlanFixedIncomePercentage
Namespace Prefix: | ggb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Allocation of plan assets, in a defined benefit plan, represented by other percentages. + References No definition available. + Details
Name: | ggb_AllocationForPlanOthersPercentage
Namespace Prefix: | ggb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Allocation of plan assets, in a defined benefit plan, represented as a percentage. + References No definition available. + Details
Name: | ggb_AllocationForPlanPercentage
Namespace Prefix: | ggb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Allocation of plan assets, in a defined benefit plan, represented by variable income percentage. + References No definition available. + Details
Name: | ggb_AllocationForPlanVariableIncomePercentage
Namespace Prefix: | ggb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_StatementGeographicalAxis=country_BR
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementGeographicalAxis=srt_NorthAmericaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_DefinedBenefitPlansAxis=ifrs-full_DefinedBenefitPlansOtherThanMultiemployerPlansStatePlansAndPlansThatShareRisksBetweenEntitiesUnderCommonControlMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |