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Bank borrowings - Borrowing detail by currency - (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Borrowings [abstract] |  |
Non-Current Principal Amount | $ 135,919 |
Current Principal Amount | 7,698 | $ 1,003
Total | 143,617 | 1,003
US Dollars |  |
Borrowings [abstract] |  |
Non-Current Principal Amount | 78,664 |
Current Principal Amount | 785 | 992
Total | 79,449 | 992
Euros |  |
Borrowings [abstract] |  |
Non-Current Principal Amount | 57,255 |
Current Principal Amount | 6,913 |
Total | $ 64,168 |
Other |  |
Borrowings [abstract] |  |
Current Principal Amount |  | 11
Total |  | $ 11

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- Definition Borrowings, Gross + References No definition available. + Details
Name: | gsm_BorrowingsGross
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The amount of long term debt before accrued charges and issuance costs. + References No definition available. + Details
Name: | gsm_LongtermBorrowingsGross
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Short-term Borrowings, Gross + References No definition available. + Details
Name: | gsm_ShortTermBorrowingsGross
Namespace Prefix: | gsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Details
Name: | srt_CurrencyAxis=currency_USD
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_CurrencyAxis=currency_EUR
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_CurrencyAxis=gsm_CurrencyOtherThanUsdMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |