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Segment reporting - Sales by product line (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of products and services [abstract] |  |  |  |  |
Sales | $ 2,274,038 | $ 1,741,693 | [1] | $ 1,576,037 | [1]
Silicon Metal Product Line [Member] |  |  |  |  |
Disclosure of products and services [abstract] |  |  |  |  |
Sales | 933,366 | 739,618 |  | 751,508 |
Manganese Alloys Product Line [Member] |  |  |  |  |
Disclosure of products and services [abstract] |  |  |  |  |
Sales | 527,757 | 363,644 |  | 223,451 |
Ferrosilicon Product Line [Member] |  |  |  |  |
Disclosure of products and services [abstract] |  |  |  |  |
Sales | 359,374 | 266,862 |  | 242,788 |
Other Silicon Based Alloys Product Line [Member] |  |  |  |  |
Disclosure of products and services [abstract] |  |  |  |  |
Sales | 215,697 | 188,183 |  | 173,901 |
Silica Fume Product Line [Member] |  |  |  |  |
Disclosure of products and services [abstract] |  |  |  |  |
Sales | 37,061 | 36,338 |  | 37,480 |
Energy Product Line [Member] |  |  |  |  |
Disclosure of products and services [abstract] |  |  |  |  |
Sales | 44,185 | 16,661 |  | 20,380 |
Other Product Lines [Member] |  |  |  |  |
Disclosure of products and services [abstract] |  |  |  |  |
Sales | $ 156,598 | $ 130,387 |  | $ 126,529 |
[1] | The amounts for 2016 have been re-presented to show the results of the Spanish energy business within profit (loss) from continuing operations, as described in Note 1 to the consolidated financial statements.

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- Definition The amount of revenue from contracts with customers. A customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration. [Refer: Revenue] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2018-01-01 -Paragraph 113 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=15&date=2018-03-01&anchor=para_113_a&doctype=Standard -URIDate 2018-03-16 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IFRS -Number 15 -IssueDate 2018-01-01 -Paragraph 114 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=15&date=2018-03-01&anchor=para_114&doctype=Standard -URIDate 2018-03-16 + Details
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