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CONSOLIDATED FINANCIAL STATEMENTS - Gains and losses on assets held for sale and sale of interest in subsidiaries and associate company (Details) T in Thousands, € in Millions, R$ in Millions, $ in Millions |  | 3 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 29, 2017 BRL (R$) | Jun. 30, 2016 EUR (€) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 BRL (R$) | Dec. 31, 2016 BRL (R$) | Dec. 31, 2017 EUR (€) | Dec. 31, 2017 USD ($) | Dec. 31, 2017 BRL (R$) | Dec. 29, 2017 USD ($) T | Dec. 29, 2017 BRL (R$) T | Oct. 10, 2017 USD ($) T | Oct. 10, 2017 BRL (R$) T | Dec. 31, 2016 EUR (€) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 BRL (R$) | Jun. 30, 2016 BRL (R$)
Chilean operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent of operations contracted for sale |  |  |  |  |  |  |  |  |  |  | 100.00% | 100.00% |  |  |  |
Economic value of transaction |  |  |  |  |  |  |  |  |  |  | $ 154.0 | R$ 509.0 |  |  |  |
Chilean operations | Steel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Production capacity (in tons) | T |  |  |  |  |  |  |  |  |  |  | 520 | 520 |  |  |  |
Certain operations in the U.S. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income (expense) recognized as a result of operations with subsidiaries, associates and joint entities | R$ (649.2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Consideration agreement amount |  |  |  |  |  |  |  |  | $ 600.0 | R$ 1,985.0 |  |  |  |  |  |
Certain operations in the U.S. | Steel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Production capacity (in tons) | T |  |  |  |  |  |  |  |  | 2,500 | 2,500 |  |  |  |  |  |
Discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in indebtedness |  |  |  |  | R$ (291.0) |  |  |  |  |  |  |  |  |  |  |
Decrease in working capital |  |  |  |  | (438.0) |  |  |  |  |  |  |  |  |  |  |
Gerdau Holdings Europa S.A. and subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Enterprise value of the transaction |  | € 155.0 |  |  |  |  |  |  |  |  |  |  |  |  |  | R$ 621.0
Maximum additional contingent consideration receivable |  | € 45.0 |  |  |  |  |  |  |  |  |  |  |  |  |  | R$ 180.0
Period of time over which contingent consideration may be received |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivable amount in sale of business |  |  |  |  |  | € 28.9 |  | R$ 121.0 |  |  |  |  | € 32.5 |  | R$ 112.0 |
Income (expense) recognized as a result of operations with subsidiaries, associates and joint entities |  |  |  |  | (105.0) |  |  |  |  |  |  |  |  |  |  |
Corporacin Centroamericana del Acero S.A. and subsidiaries and Cleary Holdings Corp. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Receivable amount in sale of business |  |  |  |  |  |  | $ 16.0 | R$ 52.0 |  |  |  |  |  | $ 79.0 | 257.0 |
Income (expense) recognized as a result of operations with subsidiaries, associates and joint entities |  |  |  |  | R$ 47.0 |  |  |  |  |  |  |  |  |  |  |
Corporacin Centro Americana del Acero S.A. and subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Enterprise value of the transaction |  |  |  |  |  |  |  |  |  |  |  |  |  | 70.0 | 222.7 |
Cleary Holdings Corp. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Enterprise value of the transaction |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30.2 | R$ 102.6 |
Gerdau Summit Aos Fundidos e Forjados S.A. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Results in operations with subsidiaries and associate company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash received |  |  | $ 44.7 | R$ 147.9 |  |  |  |  |  |  |  |  |  |  |  |
Fair value adjustment |  |  |  | R$ (72.5) |  |  |  |  |  |  |  |  |  |  |  |

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