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v3.19.1

Guarantee commitments to third parties and contingent liabilities (Details) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) item employee case | Dec. 31, 2017 USD ($)
Guarantee commitments to third parties and other contingent assets and liabilities |  |
Bank guarantees commitments to third parties | $ | $ 14,427 | $ 18,943
Number of employees | employee | 93 |
Number of case closed | case | 61 |
Number of faute inexcusable | item | 13 |
Statutory maximum penalty per day per violation of standard provisions | $ | $ 93,750 |
Asbestos-related claims |  |
Guarantee commitments to third parties and other contingent assets and liabilities |  |
Number of claims pending | item | 32 |
Number of grave cases | item | 3 |
Bond to guarantee civil financial responsibility | $ | $ 1,775 |

X
- Definition The amount of bank guarantees commitments to third party. + References No definition available. + Details
Name: | gsm_BankGuaranteesCommitmentsToThirdParties
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of bond to guarantee civil financial responsibility + References No definition available. + Details
Name: | gsm_BondToGuaranteeCivilFinancialResponsibility
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Number of claims settled. + References No definition available. + Details
Name: | gsm_IfrsLossContingencyClaimsSettledNumber
Namespace Prefix: | gsm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of plaintiffs that have filed claims pertaining to a loss contingency. + References No definition available. + Details
Name: | gsm_IfrsLossContingencyNumberOfPlaintiffs
Namespace Prefix: | gsm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of pending claims pertaining to a loss contingency. + References No definition available. + Details
Name: | gsm_IfrsLossContingencyPendingClaimsNumber
Namespace Prefix: | gsm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of inexcusable negligence pertaining to a loss contingency. + References No definition available. + Details
Name: | gsm_LossContingencyNumberOfInexcusableNegligence
Namespace Prefix: | gsm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of grave cases that have been litigated through the appeal process and in which claimants’ assertions of inexcusable negligence were upheld. + References No definition available. + Details
Name: | gsm_NumberOfGraveCases
Namespace Prefix: | gsm_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of statutory maximum penalty for violation of standard provisions per day. + References No definition available. + Details
Name: | gsm_StatutoryMaximumPenaltyForViolationOfStandardProvisions
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
Name: | ifrs-full_DisclosureOfContingentLiabilitiesLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | ifrs-full_ClassesOfContingentLiabilitiesAxis=gsm_LegalProceedingsAsbestosClaimsContingentLiabilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |