Name:	ggb_ActuarialGainsLossesArisingFromExperienceAdjustmentsOnDefinedBenefitLiabilities
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The decrease (increase) in plan assets resulting from actuarial gains (losses) arising from experience adjustments that result in fair value remeasurements of plan assets. Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred.

No definition available.

Name:	ggb_ActuarialGainsLossesArisingFromExperienceAdjustmentsOnPlanAssets
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

Name:	ggb_HistoricalActuarialRemeasurementsAbstract
Namespace Prefix:	ggb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_57_a&doctype=Standard
-URIDate 2018-03-16

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

The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

Name:	ifrs-full_PlanAssetsAtFairValue
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The fair value of any plan assets, less the present value of the defined benefit obligation. [Refer: Plan assets [member]]