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CONSOLIDATED FINANCIAL STATEMENTS - Joint ventures (Details) T in Thousands, R$ in Thousands, $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2017 BRL (R$) | Dec. 31, 2017 BRL (R$) | Dec. 31, 2016 BRL (R$) | Dec. 31, 2015 BRL (R$) | Jun. 30, 2017 USD ($) T | Jun. 30, 2017 BRL (R$) T
Joint ventures |  |  |  |  |  |
Net income (loss) |  | R$ (338,667) | R$ (2,885,929) | R$ (4,595,986) |  |
Total comprehensive income |  | (298,366) | (7,773,542) | R$ 1,094,348 |  |
Joint ventures |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Net income (loss) |  | (91,553) | (126,723) |  |  |
Total comprehensive income |  | R$ (91,553) | R$ (126,723) |  |  |
Bradley Steel Processors |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Equity interest in joint venture |  | 50.00% | 50.00% | 50.00% |  |
MRM Guide Rail |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Equity interest in joint venture |  | 50.00% | 50.00% | 50.00% |  |
Gerdau Corsa S.A.P.I de C.V. |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Equity interest in joint venture |  | 50.00% | 50.00% | 50.00% |  |
Gerdau Metaldom Corp. |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Equity interest in joint venture |  | 45.00% | 45.00% | 45.00% |  |
Gerdau Summit Aos Fundidos e Forjados S.A. |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Equity interest in joint venture |  | 58.73% |  |  |  |
Diaco S.A. |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Equity interest in joint venture | 50.00% | 49.87% |  |  |  |
Economic value of transaction |  |  |  |  | $ 165 | R$ 546,000
Decrease in indebtedness | R$ (226,000) |  |  |  |  |
Decrease in working capital | R$ (175,000) |  |  |  |  |
Diaco S.A. | Steel |  |  |  |  |  |
Joint ventures |  |  |  |  |  |
Production capacity (in tons) | T |  |  |  |  | 674 | 674

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