DEBENTURES - Summary (Details) R$ in Thousands	12 Months Ended
Dec. 31, 2019 BRL (R$) instrument	Dec. 31, 2018 BRL (R$)
DEBENTURES
Total Debentures	R$ 2,911,044	R$ 1,538,873
Current debentures	18,015	2,755
Non-current debentures	R$ 2,893,029	1,536,118
3rd- A and B
DEBENTURES
Total Debentures		18,871
7th
DEBENTURES
Total Debentures		1,102
8th
DEBENTURES
Total Debentures		8,080
9th
DEBENTURES
Total Debentures		2,349
11th - A and B
DEBENTURES
Total Debentures		5,716
15th
DEBENTURES
Quantity Issued \| instrument	1,500,000
Total Debentures	R$ 1,504,436	R$ 1,502,755
16th - A
DEBENTURES
Quantity Issued \| instrument	600,000
Total Debentures	R$ 602,847
16th - B
DEBENTURES
Quantity Issued \| instrument	800,000
Total Debentures	R$ 803,761

The number of debentures issued.

No definition available.

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

- Definition

The amount of current notes and debentures issued and the 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_CurrentNotesAndDebenturesIssuedAndCurrentPortionOfNoncurrentNotesAndDebenturesIssued
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

+ 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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	ifrs-full_BorrowingsByNameAxis=ggb_DebenturesIssuanceFifteenthMember
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: