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v3.19.1

Non-current assets held for sale - (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($)
Held for sale |
Current liabilities |
Adjustment to the carrying amount of its assets equivalent to the depreciation that would have been charged if the business had not been classified as held for sale | $ 2,608

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- Definition The amount of adjustment to the carrying amount of its assets equivalent to the depreciation that would have been charged if the business had not been classified as held for sale. + References No definition available. + Details
Name: | gsm_AdjustmentToCarryingAmountOfAssets
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Details
Name: | ifrs-full_AssetsAndLiabilitiesClassifiedAsHeldForSaleAxis=ifrs-full_AssetsAndLiabilitiesClassifiedAsHeldForSaleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |