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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 |  |  |  |  |
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Nov. 05, 2018 USD ($) item | Oct. 31, 2018 USD ($) | Jul. 31, 2018 BRL (R$) Plant | Jun. 29, 2018 USD ($) Plant | Jun. 30, 2017 USD ($) | Dec. 31, 2018 BRL (R$) | Dec. 31, 2017 BRL (R$) | Nov. 05, 2018 BRL (R$) | Oct. 31, 2018 BRL (R$) | Jun. 29, 2018 BRL (R$) | Mar. 30, 2018 USD ($) | Mar. 30, 2018 BRL (R$) | Jun. 30, 2017 BRL (R$)
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Gains and losses on assets held for sale and sales of interest in subsidiaries | R$ |  |  |  |  |  | R$ (414,507) | R$ (721,682) |  |  |  |  |  |
Wire rod mill and processing units |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Enterprise value of the transaction |  |  |  |  |  |  |  |  |  |  | $ 99.5 | R$ 330,700 |
Hydroelectric plants |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Enterprise value of the transaction | R$ |  |  | R$ 835,000 |  |  |  |  |  |  |  |  |  |
Number of hydroelectric plants | Plant |  |  | 2 |  |  |  |  |  |  |  |  |  |
Rebar-producing mills |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Enterprise value of the transaction | $ 600.0 |  |  |  |  |  |  | R$ 2,222,900 |  |  |  |  |
Number of rebar-producing mills | item | 4 |  |  |  |  |  |  |  |  |  |  |  |
Aza Participaes SpA and Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Proportion of ownership interest in subsidiary sold |  |  |  | 100.00% |  |  |  |  |  |  |  |  |
Number of production plants | Plant |  |  |  | 3 |  |  |  |  |  |  |  |  |
Economic value of transaction |  |  |  | $ 154.1 |  |  |  |  |  | R$ 594,200 |  |  |
Gerdau Hungria KFT Y CIA Sociedad Regular Colectiva |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Enterprise value of the transaction |  | $ 120.0 |  |  |  |  |  |  | R$ 490,200 |  |  |  |
Proportion of ownership interest in subsidiary sold |  | 98.89% |  |  |  |  |  |  |  |  |  |  |
Percentage of Perimeter |  | 100.00% |  |  |  |  |  |  |  |  |  |  |
Diaco S.A. |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Proportion of ownership interest in subsidiary sold |  |  |  |  | 50.00% |  |  |  |  |  |  |  |
Economic value of transaction |  |  |  |  | $ 165.0 |  |  |  |  |  |  |  | R$ 546,000
Diaco S.A. |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |
Proportion of ownership interest in joint venture |  |  |  |  | 49.86% |  |  |  |  |  |  |  |

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