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CONSOLIDATED FINANCIAL STATEMENTS - Gains and losses on assets held for sale and sale of interest in subsidiaries (Details) R$ in Thousands, $ in Millions |  |  |  |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 05, 2018 USD ($) item | Oct. 31, 2018 USD ($) | Jul. 31, 2018 BRL (R$) item | Jun. 29, 2018 USD ($) Plant | Mar. 30, 2018 USD ($) | Mar. 30, 2018 BRL (R$) | Dec. 31, 2020 BRL (R$) | Dec. 31, 2019 BRL (R$) | Dec. 31, 2018 BRL (R$) | Nov. 05, 2018 BRL (R$) | Oct. 31, 2018 BRL (R$) | Jun. 29, 2018 BRL (R$)
Gains and losses on assets held for sale and sales of interest in subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Expenses attributable to assets held for sale and sales of interest in subsidiaries | R$ |  |  |  |  |  |  | R$ 0 | R$ 0 | R$ 414,507 |  |  |
Optimus Steel LLC |  |  |  |  |  |  |  |  |  |  |  |
Gains and losses on assets held for sale and sales of interest in subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale |  |  |  |  | $ 99.5 | R$ 330,700 |  |  |  |  |  |
Aza Participacoes SpA and subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Gains and losses on assets held for sale and sales of interest in subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Proportion of ownership interest in subsidiary sold |  |  |  | 100.00% |  |  |  |  |  |  |  |
Number of production plants | Plant |  |  |  | 3 |  |  |  |  |  |  |  |
Total value of sale |  |  |  | $ 154.1 |  |  |  |  |  |  |  | R$ 594,200
Cacu and Barra dos Coqueiros hydroelectric plants |  |  |  |  |  |  |  |  |  |  |  |
Gains and losses on assets held for sale and sales of interest in subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Number of hydroelectric plants | item |  |  | 2 |  |  |  |  |  |  |  |  |
Proceeds from sale | R$ |  |  | R$ 835,000 |  |  |  |  |  |  |  |  |
Gerdau Hungria KFT Y CIA Sociedad Regular Colectiva |  |  |  |  |  |  |  |  |  |  |  |
Gains and losses on assets held for sale and sales of interest in subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Proportion of ownership interest in subsidiary sold |  | 98.89% |  |  |  |  |  |  |  |  |  |
Total value of sale |  | $ 120.0 |  |  |  |  |  |  |  |  | R$ 490,200 |
Gerdau Hungria KFT Y CIA Sociedad Regular Colectiva | INDIA |  |  |  |  |  |  |  |  |  |  |  |
Gains and losses on assets held for sale and sales of interest in subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Proportion of ownership interest in subsidiary sold |  | 100.00% |  |  |  |  |  |  |  |  |  |
Sale of production units and distribution centers in the U.S. |  |  |  |  |  |  |  |  |  |  |  |
Gains and losses on assets held for sale and sales of interest in subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Number of rebar-producing mills | item | 4 |  |  |  |  |  |  |  |  |  |  |
Total value of sale | $ 600.0 |  |  |  |  |  |  |  |  | R$ 2,222,900 |  |

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