EMPLOYEE BENEFITS - Defined benefit pension plan - Allocation of plan assets (Details)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
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	54.90%	58.10%
Variable income	42.20%	36.50%
Others	2.90%	5.40%
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%

Target allocation range in alternative securities, in a defined benefit plan, represented as a percentage.

No definition available.

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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

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- Definition

Target allocation range in shares, in a defined benefit plan, represented as a percentage.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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- Definition

Allocation of plan assets, in a defined benefit plan, represented by fixed income percentage.

+ References

No definition available.

+ Details

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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
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Period Type:	duration

- 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
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Period Type:	duration

- Definition

Allocation of plan assets, in a defined benefit plan, represented by variable income percentage.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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- Details

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- Details

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