CONSOLIDATED FINANCIAL STATEMENTS - Acquisition of entity (Details) - Gerdau Acos Longos S.A. - Hierros Anon, SA and Gallega de Mallas, SL [Member] R$ in Millions, $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)	Dec. 31, 2019 BRL (R$)	Nov. 26, 2019
Disclosure Of Investment Entities [Line Items]
Percentage of equity interest acquired (as a percent)			96.35%
Economic value	$ 110.8	R$ 446.6

The percentage of voting equity interests acquired in a business combination. [Refer: Business combinations [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 3
-IssueDate 2019-01-01
-Paragraph B64
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=3&code=ifrs-tx-2019-en-r&anchor=para_B64_c&doctype=Appendix&subtype=B
-URIDate 2019-03-27

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