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

- 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

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

- Definition

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

+ References

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

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

Bank borrowings - Other Loans - (Details) - USD ($) $ in Thousands	Dec. 10, 2019	Dec. 31, 2019	Dec. 09, 2019	Dec. 31, 2018
Bank Borrowings
Outstanding balance		$ 158,999		$ 141,012
Ferrous Receivables DAC \| ING \| Securitizations
Bank Borrowings
Outstanding balance			$ 75,000
Ferrous Receivables DAC \| New senior loans \| Securitizations
Bank Borrowings
Gross loan commitments	$ 150,000
Outstanding balance		$ 104,130
Term of the securitization program	2 years