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Equity - Valuation adjustments (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Equity [abstract] |  |
Actuarial gains and losses | $ (390) | $ (2,998)
Hedging instruments and other | (11,169) | (13,801)
Valuation adjustments | $ (11,559) | $ (16,799)

X
- Definition Total amount of valuation adjustments as of the period end. + References No definition available. + Details
Name: | gsm_AccumulatedValuationAdjustments
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The amount of actuarial gains and losses. + References No definition available. + Details
Name: | gsm_ActuarialGainLossValuationAdjustment
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount valuation adjustments for hedging instruments and other. + References No definition available. + Details
Name: | gsm_HedgingInstrumentAndOtherValuationAdjustment
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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