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Non-current assets held for sale - (Details) $ in Thousands	12 Months Ended
	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: