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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

X

- 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