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IMPAIRMENT OF ASSETS (Details) R$ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 BRL (R$) | Dec. 31, 2018 BRL (R$) segment | Dec. 31, 2017 BRL (R$) | Dec. 31, 2016 BRL (R$)
Impairment of assets |  |  |  |  |
Total impairment of non-financial assets |  |  |  | R$ 1,114,807 | R$ 2,917,911
Impairment of property, plant and equipment |  | R$ 265,369 | R$ 0 | 265,369 |
Impairment of goodwill |  |  | R$ 0 | R$ 849,438 | R$ 2,678,582
Number of operating segments | segment |  |  | 4 |  |
Period for projecting the cash flows for the goodwill impairment test | P5Y |  |  |  |
Discount rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, percent of increase |  | 0.50% | 0.50% | 0.50% |
Perpetuity growth rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, percent of decrease |  | 0.50% | 0.50% | 0.50% |
North America |  |  |  |  |
Impairment of assets |  |  |  |  |
Impairment of property, plant and equipment |  | R$ 220,090 |  |  |
Impairment of goodwill |  | R$ 849,438 |  |  |
Perpetuity growth rate |  | 3.00% | 3.00% | 3.00% |
Pre-tax discount rate |  | 13.60% | 13.80% | 13.60% |
Amount by which unit's recoverable amount is (below) exceeds its carrying value |  | R$ (849,000) | R$ 2,157 | R$ (849,000) |
North America | Discount rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | (506,000) | 1,474,000 | (506,000) |
North America | Perpetuity growth rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | R$ (310,000) | R$ 1,672,000 | R$ (310,000) |
South America |  |  |  |  |
Impairment of assets |  |  |  |  |
Perpetuity growth rate |  | 3.00% | 3.00% | 3.00% |
Pre-tax discount rate |  | 14.70% | 17.10% | 14.70% |
Amount by which unit's recoverable amount is (below) exceeds its carrying value |  | R$ 437,000 | R$ 730,000 | R$ 437,000 |
South America | Discount rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | 265,000 | 593,000 | 265,000 |
South America | Perpetuity growth rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | 318,000 | R$ 640,000 | R$ 318,000 |
Brazil |  |  |  |  |
Impairment of assets |  |  |  |  |
Impairment of property, plant and equipment |  | R$ 45,279 |  |  |
Perpetuity growth rate |  | 3.00% | 3.00% | 3.00% |
Pre-tax discount rate |  | 15.20% | 16.40% | 15.20% |
Amount by which unit's recoverable amount is (below) exceeds its carrying value |  | R$ 1,747,000 | R$ 3,924,000 | R$ 1,747,000 |
Brazil | Discount rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | 999,000 | 3,048,000 | 999,000 |
Brazil | Perpetuity growth rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | R$ 1,254,000 | R$ 3,318,000 | R$ 1,254,000 |
Special Steel |  |  |  |  |
Impairment of assets |  |  |  |  |
Perpetuity growth rate |  | 3.00% | 3.00% | 3.00% |
Pre-tax discount rate |  | 13.10% | 14.70% | 13.10% |
Amount by which unit's recoverable amount is (below) exceeds its carrying value |  | R$ 3,989,000 | R$ 3,019,000 | R$ 3,989,000 |
Special Steel | Discount rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | 3,422,000 | 2,483,000 | 3,422,000 |
Special Steel | Perpetuity growth rate |  |  |  |  |
Impairment of assets |  |  |  |  |
Sensitivity analysis, amount above (below) book value |  | R$ 3,596,000 | R$ 2,645,000 | R$ 3,596,000 |

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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
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