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Earnings (loss) per ordinary share - Antidilutive (Details) - shares | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Earnings per share [abstract] |  |  |
Potential ordinary shares excluded from calculation of diluted (loss) earnings per share because their effect would be anti-dilutive | 269,116 | 70,673 | 96,236

X
- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References No definition available. + Details
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Namespace Prefix: | gsm_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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