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

Expected amount of decrease in other operating expenses

No definition available.

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

Amount of expected increase in the depreciation expense.

No definition available.

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

Amount of expected increase in the finance costs

No definition available.

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

The percentage of ownership interest held by related party.

No definition available.

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

The interest rate on borrowings. [Refer: Borrowings]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 7
-IssueDate 2018-01-01
-Paragraph 7
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_7&doctype=Standard
-URIDate 2018-03-16

Name:	ifrs-full_BorrowingsInterestRate
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
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_DescriptionOfExpectedImpactOfInitialApplicationOfNewStandardsOrInterpretationsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The amount of liabilities related to the entity's leases. Lease is a contract, or part of a contract, that conveys the right to use an underlying asset for a period of time in exchange for consideration.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2019-01-01
-Name IFRS
-Number 16
-IssueDate 2018-01-01
-Paragraph 47
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=16&date=2018-03-01&anchor=para_47_b&doctype=Standard
-URIDate 2018-03-16

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

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2018-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_112_c&doctype=Standard
-URIDate 2018-03-16