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Goodwill - Changes in carrying amount and impairment testing (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Reconciliation of changes in goodwill [abstract] |  |  |
Goodwill at beginning of period | $ 205,287 | $ 230,210 |
Impairment |  | (30,618) | $ (194,612)
Exchange differences | (2,439) | 5,695 |
Goodwill at end of period | 202,848 | 205,287 | 230,210
Electrometallurgy - North America |  |  |
Reconciliation of changes in goodwill [abstract] |  |  |
Goodwill at beginning of period | 205,287 |  |
Goodwill at end of period | 202,848 | 205,287 |
Electrometallurgy United States Segment |  |  |
Reconciliation of changes in goodwill [abstract] |  |  |
Goodwill at end of period | 172,900 |  |
Electrometallurgy Canada Segment |  |  |
Reconciliation of changes in goodwill [abstract] |  |  |
Goodwill at end of period | 29,900 |  |
Globe Specialty Metals Inc |  |  |
Reconciliation of changes in goodwill [abstract] |  |  |
Goodwill at beginning of period | 205,287 | 230,210 |
Impairment |  | (30,618) |
Exchange differences | (2,439) | 5,695 |
Goodwill at end of period | 202,848 | 205,287 | $ 230,210
Globe Specialty Metals Inc | Electrometallurgy United States Segment |  |  |
Reconciliation of changes in goodwill [abstract] |  |  |
Goodwill at end of period | 172,913 |  |
Globe Specialty Metals Inc | Electrometallurgy Canada Segment |  |  |
Reconciliation of changes in goodwill [abstract] |  |  |
Goodwill at end of period | $ 29,935 |  |
CANADA | Electrometallurgy - North America |  |  |
Reconciliation of changes in goodwill [abstract] |  |  |
Impairment |  | $ (30,618) |

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- Definition The amount of assets representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognised. [Refer: Business combinations [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 3 -IssueDate 2018-01-01 -Paragraph B67 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=3&date=2018-03-01&anchor=para_B67_d&doctype=Appendix&seq=2 -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 36 -IssueDate 2018-01-01 -Paragraph 134 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=36&date=2018-03-01&anchor=para_134_a&doctype=Standard -URIDate 2018-03-16 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 36 -IssueDate 2018-01-01 -Paragraph 135 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=36&date=2018-03-01&anchor=para_135_a&doctype=Standard -URIDate 2018-03-16 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 54 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_54_c&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The amount of impairment loss recognised in profit or loss for goodwill. [Refer: Impairment loss recognised in profit or loss; Goodwill] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 3 -IssueDate 2018-01-01 -Paragraph B67 -Subparagraph d -Clause v -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=3&date=2018-03-01&anchor=para_B67_d_v&doctype=Appendix&seq=2 -URIDate 2018-03-16 + Details
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- Definition The increase (decrease) in goodwill resulting from net exchange differences arising on the translation of the financial statements from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity. [Refer: Goodwill] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 3 -IssueDate 2018-01-01 -Paragraph B67 -Subparagraph d -Clause vi -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=3&date=2018-03-01&anchor=para_B67_d_vi&doctype=Appendix&seq=2 -URIDate 2018-03-16 + Details
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