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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 |

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