Per unit value of debentures issued.

No definition available.

Name:	ggb_DebentureIssuedParValuePerUnit
Namespace Prefix:	ggb_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of debentures issued.

+ References

No definition available.

+ Details

Name:	ggb_DebenturesNumberIssued
Namespace Prefix:	ggb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

No definition available.

+ Details

Name:	ggb_IfrsDebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	ggb_
Data Type:	num:percentItemType
Balance Type:	na
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

The interest rate on borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 7
-IssueDate 2019-01-01
-Paragraph 7
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_7&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- 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 non-current portion of non-current notes and debentures issued. [Refer: Notes and debentures issued]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The amount of notes and debentures issued by the entity.

+ References

+ Details

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

- Definition

The nominal value per share.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 79
-Subparagraph a
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_79_a_iii&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_ParValuePerShare
Namespace Prefix:	ifrs-full_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_MaturityAxis=ifrs-full_NotLaterThanOneYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_MaturityAxis=ifrs-full_LaterThanOneYearAndNotLaterThanTwoYearsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_MaturityAxis=ifrs-full_LaterThanTwoYearsAndNotLaterThanThreeYearsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_MaturityAxis=ifrs-full_LaterThanThreeYearsAndNotLaterThanFourYearsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_MaturityAxis=ifrs-full_LaterThanFourYearsAndNotLaterThanFiveYearsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_MaturityAxis=ifrs-full_LaterThanFiveYearsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

DEBENTURES - Amortization schedules (Details)		12 Months Ended
DEBENTURES - Amortization schedules (Details)	Apr. 25, 2019 BRL (R$) R$ / shares	Dec. 31, 2019 BRL (R$) instrument	Dec. 31, 2018 BRL (R$)
DEBENTURES
Debentures		R$ 2,911,044,000	R$ 1,538,873,000
Non-current portion of non-current notes and debentures issued		2,893,029,000	1,536,118,000
Unit par value \| R$ / shares	R$ 1,000.00
Next 12 months
DEBENTURES
Non-current portion of non-current notes and debentures issued			5,716,000
Year Two
DEBENTURES
Non-current portion of non-current notes and debentures issued			18,871,000
Year Three
DEBENTURES
Non-current portion of non-current notes and debentures issued		1,496,591,000	1,501,102,000
Year Four
DEBENTURES
Non-current portion of non-current notes and debentures issued		598,530,000	8,080,000
Year Five
DEBENTURES
Non-current portion of non-current notes and debentures issued			R$ 2,349,000
More than 5 years
DEBENTURES
Non-current portion of non-current notes and debentures issued		R$ 797,908,000
Debentures
DEBENTURES
Nominal annual interest rate (as a percent)		6.18%	6.60%
16th
DEBENTURES
Debentures	R$ 1,400,000,000,000
Quantity Issued	600
Par value per debenture \| R$ / shares	R$ 800.00
16th - A
DEBENTURES
Debentures		R$ 602,847,000
Quantity Issued \| instrument		600,000
Maturity period	48 months
Yield interest percent	105.50%
16th - B
DEBENTURES
Debentures		R$ 803,761,000
Quantity Issued \| instrument		800,000
Maturity period	84 months
Yield interest percent	107.25%