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

- References

No definition available.

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

- References

No definition available.

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

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c_iii&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_ActuarialGainsLossesArisingFromChangesInFinancialAssumptionsNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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 2017-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_57_a&doctype=Standard
-URIDate 2017-03-09

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

The amount of other comprehensive income, before tax, related to gains (losses) on remeasurements of defined benefit plans, which comprise actuarial gains and losses; the return on plan assets, excluding amounts included in net interest on the net defined benefit liability (asset); and any change in the effect of the asset ceiling, excluding amounts included in net interest on the net defined benefit liability (asset). [Refer: Other comprehensive income, before tax; Defined benefit plans [member]; Plan assets [member]; Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 91
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_91_b&doctype=Standard
-URIDate 2017-03-09

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 7
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_7&doctype=Standard
-URIDate 2017-03-09

Name:	ifrs-full_OtherComprehensiveIncomeBeforeTaxGainsLossesOnRemeasurementsOfDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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