Earnings (loss) per ordinary share - Antidilutive (Details) - shares	12 Months Ended
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

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.

No definition available.

Name:	gsm_IfrsAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	gsm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration