The amount of outstanding funds that the entity is obligated to repay, excluding notes and debentures.

No definition available.

Name:	ggb_BorrowingsExcludingNotesAndDebentures
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	ggb_IfrsDebtInstrumentTerm
Namespace Prefix:	ggb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

No definition available.

+ Details

Name:	ggb_IfrsLineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	ggb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The number of tranches under the credit agreement.

+ References

No definition available.

+ Details

Name:	ggb_NumberOfTranches
Namespace Prefix:	ggb_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

The adjustment to the basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 7
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_7&doctype=Standard
-URIDate 2017-03-09

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The interest rate on borrowings. [Refer: Borrowings]

+ References

+ Details

Name:	ifrs-full_BorrowingsInterestRate
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings]

+ References

+ Details

Name:	ifrs-full_BorrowingsInterestRateBasis
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfDetailedInformationAboutBorrowingsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of financial assets that the entity has pledged as collateral for liabilities or contingent liabilities, including amounts that have been reclassified in accordance with paragraph 3.2.23(a) of IFRS 9. [Refer: Contingent liabilities [member]; Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2017-01-01
-Paragraph 14
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_14_a&doctype=Standard
-URIDate 2017-03-09

+ Details

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

- Definition

The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2017-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2017-03-09

+ Details

Name:	ifrs-full_NotionalAmount
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_BorrowingsByNameAxis=ggb_TenYearBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=ggb_GerdauTradeIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=ggb_FinameBndesProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=ggb_BndesLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=ggb_SeniorUnsecuredGlobalWorkingCapitalCreditAgreementLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=ggb_GerdausNorthAmericanSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=ggb_GerdausLatinAmericanSubsidiariesMember
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LOANS AND FINANCING - Additional Disclosures (Details) R$ in Thousands, $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2017 BRL (R$)	Dec. 31, 2017 USD ($) tranche	Dec. 31, 2017 BRL (R$) tranche	Dec. 31, 2016 BRL (R$)	Jun. 30, 2009 BRL (R$)
LOANS AND FINANCING
Notional value			R$ 16,207,120	R$ 20,049,854
Loans and financing			16,461,656	R$ 20,417,810
Ten Year Bonds \| Gerdau Trade Inc.
LOANS AND FINANCING
Notional value	R$ 650,000
Debt instrument term	10 years
Coupon rate	4.875%
FINAME/BNDES program
LOANS AND FINANCING
Loan amount guaranteed by asset being financed			R$ 110,400
Credit line with BNDES
LOANS AND FINANCING
Maximum borrowing capacity					R$ 1,500,000
Interest rate basis		TJLP
Adjustment to interest rate basis (as a percent)		2.16%	2.16%
Loans and financing			R$ 402,500
Senior Unsecured Global Working Capital Credit Agreement
LOANS AND FINANCING
Maximum borrowing capacity		$ 800.0	R$ 2,646,400
Number of tranches \| tranche		2	2
Loans and financing		$ 54.0	R$ 178,600
Senior Unsecured Global Working Capital Credit Agreement \| Gerdau's North American subsidiaries
LOANS AND FINANCING
Maximum borrowing capacity		200.0	661,600
Senior Unsecured Global Working Capital Credit Agreement \| Gerdau's Latin American subsidiaries
LOANS AND FINANCING
Maximum borrowing capacity		$ 600.0	R$ 2,028,000