EMPLOYEE BENEFITS - Defined benefit pension plan - Actuarial gains and losses (Details) - Defined benefit pension plan - BRL (R$) R$ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized as actuarial gains and losses in the statement of comprehensive income
Restriction recognized in Other Comprehensive Income	R$ (1,696)	R$ 3,065	R$ (44,453)
Remeasurements recognized in Other Comprehensive Income	1,639	80,817	(11,927)
Fair value of plan assets
Amounts recognized as actuarial gains and losses in the statement of comprehensive income
Gain (loss) on remeasurement	(232,214)	(109,153)	235,275
Present value of the defined benefit obligations
Amounts recognized as actuarial gains and losses in the statement of comprehensive income
Gain (loss) on remeasurement	R$ 235,549	R$ 186,905	R$ (202,749)

No definition available.

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

- Definition

The amount of other comprehensive income, before tax, related to gains (losses) on restrictions on defined benefit plans.

+ References

No definition available.

+ Details

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

- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from the remeasurement of that net defined benefit liability (asset). [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2017-01-01
-Paragraph 141
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_141_c&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_GainLossOnRemeasurementOfNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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)]

+ References

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

+ Details

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

- Details

Name:	ifrs-full_DefinedBenefitPlansAxis=ggb_DefinedBenefitPensionPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_NetDefinedBenefitLiabilityAssetAxis=ifrs-full_PlanAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_NetDefinedBenefitLiabilityAssetAxis=ifrs-full_PresentValueOfDefinedBenefitObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: