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

The amount of current trade receivables before deducting allowance for doubtful accounts.

No definition available.

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

Represents the amount of fixed-rate servicing fees earned.

No definition available.

Name:	gsm_FixedServicingFeesEarned
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the length of the securitization program.

No definition available.

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

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

No definition available.

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

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

No definition available.

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

Represents the amount of additional servicing fees earned.

No definition available.

Name:	gsm_VariableServicingFeesEarned
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

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

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

The interest rate on borrowings. [Refer: Borrowings]

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

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

The amount of cash on hand and demand deposits. [Refer: Cash on hand]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 7
-IssueDate 2019-01-01
-Paragraph 45
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2019-en-r&anchor=para_45&doctype=Standard
-URIDate 2019-03-27

Name:	ifrs-full_Cash
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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.

No definition available.

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

The gain (loss) recognised on derecognised financial assets as of the date of the transfer of the assets. [Refer: Financial assets]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2019-01-01
-Paragraph 42G
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_42G_a&doctype=Standard
-URIDate 2019-03-27

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

The amount of contractual undiscounted cash flows for gross commitments to receive a loan.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 7
-IssueDate 2019-01-01
-Paragraph B11D
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_B11D_e&doctype=Appendix&subtype=B
-URIDate 2019-03-27

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

The amount of interest income on loans and receivables. [Refer: Interest income; Loans and receivables]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2021-01-01
-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-2017-en-b&anchor=para_112_c&doctype=Standard&forcepdf=true
-URIDate 2019-03-27

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

Name:	ifrs-full_SignificantInvestmentsInSubsidiariesAxis=gsm_FerrousReceivablesDacMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

Name:	ifrs-full_ContinuingInvolvementInDerecognisedFinancialAssetsByTypeOfInstrumentAxis=gsm_SeniorSubordinatedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ifrs-full_ContinuingInvolvementInDerecognisedFinancialAssetsByTypeOfInstrumentAxis=gsm_JuniorSubordinatedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ifrs-full_ContinuingInvolvementInDerecognisedFinancialAssetsByTypeOfInstrumentAxis=gsm_IngSeniorLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ifrs-full_ContinuingInvolvementInDerecognisedFinancialAssetsByTypeOfInstrumentAxis=gsm_SoundPointCapitalManagementLpSeniorLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: