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FINANCIAL INSTRUMENTS - Exchange rate risk (Details) - Foreign currency risk - R$ / $	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
FINANCIAL INSTRUMENTS
Closing foreign exchange rate	5.1967	4.0307
Percentage of devaluation of Brazilian Real against the U.S. Dollar	22.44%

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- Definition

The percentage of devaluation of the Brazilian Real against the U.S. Dollar.

+ References

No definition available.

+ Details

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Namespace Prefix:	ggb_
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Period Type:	duration

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- Definition

The spot exchange rate at the end of the reporting period. Exchange rate is the ratio of exchange for two currencies. Spot exchange rate is the exchange rate for immediate delivery.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2020-01-01

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- 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.

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- Details

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