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Financial assets and other receivables - Government Grants (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Other financial assets |  |
Income from government grants | $ 26,369 | $ 15,716
Expense related to government grants | 18,923 | 9,234
Net income (loss) related to government grants | 7,446 | $ 6,482
Carrying amount of factored receivables and the secured borrowings | $ 6,913 |

X
- Definition The carrying amount of factored receivables and the secured borrowings. + References No definition available. + Details
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Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of expenses in relation to government grants. + References No definition available. + Details
Name: | gsm_ExpensesFromGovernmentGrants
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount of net income (expenses) recognised in relation to government grants. + References No definition available. + Details
Name: | gsm_RevenueExpensesFromGovernmentGrants
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | ifrs-full_DisclosureOfFinancialAssetsAbstract
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of income recognised in relation to government grants. [Refer: Government grants] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 20 -IssueDate 2018-01-01 -Paragraph 39 -Subparagraph b -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=20&date=2018-03-01&anchor=para_39_b&doctype=Standard -URIDate 2018-03-16 + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration