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

The amount of compensation to key management personnel in the form of fixed remuneration.

No definition available.

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

The amount of fees paid or payable for the other expenses.

No definition available.

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

The amount of compensation to key management personnel in the form of variable remuneration.

No definition available.

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

The amount of key management personnel, loans and advance have been granted.

No definition available.

Name:	gsm_KeyManagementPersonnelLoansAndAdvancesGranted
Namespace Prefix:	gsm_
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_DisclosureOfTransactionsBetweenRelatedPartiesLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The amount of compensation to key management personnel. [Refer: Key management personnel of entity or parent [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2018-01-01
-Paragraph 17
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2018-03-01&anchor=para_17&doctype=Standard
-URIDate 2018-03-16

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

The amount of compensation to key management personnel in the form of share-based payments. [Refer: Key management personnel of entity or parent [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2018-01-01
-Paragraph 17
-Subparagraph e
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2018-03-01&anchor=para_17_e&doctype=Standard
-URIDate 2018-03-16

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

The amount of compensation to key management personnel in the form of termination benefits. [Refer: Termination benefits expense; Key management personnel of entity or parent [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2018-01-01
-Paragraph 17
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=24&date=2018-03-01&anchor=para_17_d&doctype=Standard
-URIDate 2018-03-16