Represents the cessation of percentage of voting interest which occur in change in control.

No definition available.

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

- Definition

Represents the change in percentage of voting stock held other than by permitted holers and its affiliates which occur in change in control.

+ References

No definition available.

+ Details

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

- Definition

Percentage price of original principal amount of debt at which the debt is required to be redeemed in the event of a change in control.

+ References

No definition available.

+ Details

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

- Definition

The amount of long term debt before accrued charges and issuance costs.

+ References

No definition available.

+ Details

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

- Definition

Percentage of invoice amount of eligible receivables

+ References

No definition available.

+ Details

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

- Definition

The percentage of ownership interest held by related party.

+ References

No definition available.

+ Details

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

- Definition

Percentage of principal and interest payments.

+ References

No definition available.

+ Details

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

- Definition

Repayment term of revolver, prior to maturity date of the senior unsecured notes.

+ References

No definition available.

+ Details

Name:	gsm_RepaymentTermOfRevolverPriorToMaturityDateOfSeniorUnsecuredNotes
Namespace Prefix:	gsm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the length of the securitization program.

+ References

No definition available.

+ Details

Name:	gsm_TermOfSecuritizationProgram
Namespace Prefix:	gsm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of trade receivables authorized to be sold under the securitization program.

+ References

No definition available.

+ Details

Name:	gsm_TradeReceivablesAuthorizedToBeSoldUnderSecuritizationProgram
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of trade receivables sold under the securitization program to the SPE.

+ References

No definition available.

+ Details

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

- Definition

The amount of upfront cash consideration provided or to be provided by the special purpose designated activity company (SPE) .

+ References

No definition available.

+ Details

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

- Definition

The amount of outstanding funds that the entity is obligated to repay.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 55
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_55&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The interest rate on borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 7
-IssueDate 2019-01-01
-Paragraph 7
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_7&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_BorrowingsInterestRate
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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

- Definition

The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_NotionalAmount
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_BorrowingsByNameAxis=gsm_CreditFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=gsm_AssetBasedRevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=gsm_SeniorNotesDue2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=gsm_SeniorNotesDue2022IssuedByFerroglobePlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_InterestRateRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_CurrencyRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_LiquidityRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_BorrowingsByNameAxis=gsm_RevolvingLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfRelatedPartiesAxis=gsm_GrupoVillarMirS.a.u.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ContinuingInvolvementInDerecognisedFinancialAssetsByTypeOfTransferAxis=ifrs-full_SecuritisationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial risk management (Details) - USD ($) $ in Thousands		1 Months Ended	8 Months Ended	12 Months Ended
	Feb. 15, 2017	Feb. 28, 2017	Sep. 04, 2019	Dec. 31, 2019	Dec. 31, 2018	Oct. 11, 2019
Financial risk management
Principal amount - Gross				$ 144,388	$ 135,919
Borrowings				158,999	141,012
Credit facilities
Financial risk management
Principal amount				100,000	250,000
Principal amount - Gross				62,835	135,919
Borrowings				58,049	133,314
ABL Revolver
Financial risk management
Borrowings				50,200		$ 4,500
Senior Notes due 2022
Financial risk management
Principal amount	$ 350,000			$ 350,000	350,000
Fixed Interest Rate	9.375%
Redemption price of notes at change of control	101.00%			101.00%
Senior Notes due 2022 issued by Ferroglobe PLC
Financial risk management
Principal amount	$ 150,000
Interest rate risk
Financial risk management
Borrowings				$ 158,999	141,012
Interest rate risk \| Credit facilities
Financial risk management
Principal amount - Gross				62,835	135,919
Foreign currency risk
Financial risk management
Percentage of Principal and Interest Payments		55.00%
Foreign currency risk \| Senior Notes due 2022
Financial risk management
Principal amount		$ 350,000
Foreign currency risk \| Senior Notes due 2022 issued by Ferroglobe PLC
Financial risk management
Principal amount				150,000
Liquidity risk
Financial risk management
Cessation of percentage of voting interest which occur in change in control	100.00%
Liquidity risk \| ABL Revolver
Financial risk management
Principal amount				$ 100,000
Repayment term of revolver				3 months
Liquidity risk \| Senior Notes due 2022
Financial risk management
Principal amount	$ 350,000
Fixed Interest Rate	9.375%
Redemption price of notes at change of control	101.00%
Liquidity risk \| Revolving Credit Facility
Financial risk management
Borrowings				$ 58,049
Grupo Villar Mir, S.A.U.
Financial risk management
Ownership interest as a percentage	53.90%			54.00%
Grupo Villar Mir, S.A.U. \| Liquidity risk
Financial risk management
Ownership interest as a percentage				54.00%
Minimum
Financial risk management
Change in percentage of voting stock held other than by permitted holders and its affiliates to occur change in control				35.00%
Minimum \| Liquidity risk
Financial risk management
Change in percentage of voting stock held other than by permitted holders and its affiliates to occur change in control	35.00%
Securitizations
Financial risk management
Trade receivables sold under securitization program			$ 1,127,000		2,059,000
Percentage Of Invoice Amount Of Eligible Receivables				98.00%
Securitizations \| Liquidity risk
Financial risk management
Upfront cash consideration				$ 58,339	$ 227,360
Term of the securitization program					2 years
Trade receivables authorized to be sold				$ 150,000
Percentage Of Invoice Amount Of Eligible Receivables					99.00%