Name:	gsm_BondsAmountContributedToFairValueOfPlanAssetsAsPercentOfTotalDefinedBenefitPlanAssets
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The amount that cash and cash equivalents constitute of the fair value of defined benefit plan assets as a percent of total defined benefit plan assets.

No definition available.

Name:	gsm_CashAndCashEquivalentsAmountContributedToFairValueOfPlanAssetsAsPercentOfTotalDefinedBenefitPlanAssets
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The increase in defined benefit plan assets due to interest income.

No definition available.

Name:	gsm_DefinedBenefitPlanInterestIncomeOnPlanAssets
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the percentage of fair value of plan assets.

No definition available.

Name:	gsm_DisclosureOfFairValueOfPlanAssetsAsPercentage
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The amount that instruments representing equity (rather than debt) constitute of the fair value of defined benefit plan assets as a percent of total defined benefit plan assets.

No definition available.

Name:	gsm_EquityInstrumentsAmountContributedToFairValueOfPlanAssetsAsPercentOfTotalDefinedBenefitPlanAssets
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

No definition available.

Name:	gsm_IfrsDefinedBenefitPlanActualReturnOnPlanAssets
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

No definition available.

Name:	gsm_IfrsDefinedBenefitPlanBenefitsPaid
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	gsm_IfrsDefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward
Namespace Prefix:	gsm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The increase (decrease) in a defined benefit obligation that would have been caused by a change in cost of medical aid that was reasonably possible at the end of the reporting period.

No definition available.

Name:	gsm_IncreaseDecreaseInDefinedBenefitObligationDueToReasonablyPossibleChangeInCostOfMedicalAid
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Increase (decrease) in defined benefit plan assets due to changes in actuarial assumptions.

No definition available.

Name:	gsm_IncreaseDecreaseInDefinedBenefitPlanAssetsDueToActuarialDifferences
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Increase (decrease) in defined plan assets due to other reasons not specifically disclosed.

No definition available.

Name:	gsm_IncreaseDecreaseInDefinedBenefitPlanAssetsDueToOther
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amount that international investments constitutes of the fair value of defined benefit plan assets as a percent of total defined benefit plan assets.

No definition available.

Name:	gsm_InternationalInvestmentsAmountContributedToFairValueOfPlanAssetsAsPercentOfTotalDefinedBenefitPlanAssets
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The amount that other types of assets not separately disclosed constitute of the fair value of defined benefit plan assets as a percent of total defined benefit plan assets.

No definition available.

Name:	gsm_OtherAssetsAmountContributedToFairValueOfPlanAssetsAsPercentOfTotalDefinedBenefitPlanAssets
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The reasonably possible percentage of the change in the cost of medical aid used to determine the defined benefit obligation.

No definition available.

Name:	gsm_PercentageOfReasonablyPossibleChangeInCostOfMedicalAid
Namespace Prefix:	gsm_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

The amount that real estate constitutes of the fair value of defined benefit plan assets as a percent of total defined benefit plan assets.

No definition available.

Name:	gsm_RealEstateAmountContributedToFairValueOfPlanAssetsAsPercentOfTotalDefinedBenefitPlanAssets
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	ifrs-full_ChangesInNetDefinedBenefitLiabilityAssetAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The increase (decrease) in the net defined benefit liability (asset) resulting from employee service in the current period. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_a&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_CurrentServiceCostNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

Name:	ifrs-full_DisclosureOfFairValueOfPlanAssetsAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The decrease (increase) in a net defined benefit liability (asset) resulting from the remeasurement of that net defined benefit liability (asset). [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_c&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_GainLossOnRemeasurementOfNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_e&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_IncreaseDecreaseThroughChangesInForeignExchangeRatesNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_b&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_InterestExpenseIncomeNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 140
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_140_a&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_LiabilityAssetOfDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 141
-Subparagraph g
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_141_g&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_PaymentsFromPlanNetDefinedBenefitLiabilityAsset
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2018-01-01
-Paragraph 57
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2018-03-01&anchor=para_57_a&doctype=Standard
-URIDate 2018-03-16