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Length of the grace period for no installments due at the beginning of the loan.

No definition available.

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n/a

No definition available.

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Represents the number of loan agreements that were entered into.

No definition available.

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Data Type:	xbrli:integerItemType
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Period Type:	instant

Represents the number of instalments that the loan is to be repaid.

No definition available.

Name:	gsm_NumberOfLoanInstallmentsPayable
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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The interest rate on borrowings. [Refer: Borrowings]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-IssueDate 2019-01-01
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- References

No definition available.

Name:	ifrs-full_CategoriesOfFinancialLiabilitiesAbstract
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The amount of financial liabilities at amortised cost. The amortised cost is the amount at which financial liabilities are measured at initial recognition minus principal repayments, plus or minus the cumulative amortisation using the effective interest method of any difference between that initial amount and the maturity amount. [Refer: Financial liabilities]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount of loans received.

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
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Name:	ifrs-full_LoansReceived
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount of current financial liabilities that the entity does not separately disclose in the same statement or note. [Refer: Other financial liabilities; Current financial liabilities]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Name:	ifrs-full_OtherCurrentFinancialLiabilities
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount of financial liabilities that the entity does not separately disclose in the same statement or note. [Refer: Financial liabilities]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
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Name:	ifrs-full_OtherFinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The amount of non-current financial liabilities that the entity does not separately disclose in the same statement or note. [Refer: Other financial liabilities]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2019-01-01
-Paragraph B13
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_54_m&doctype=Standard
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Name:	ifrs-full_OtherNoncurrentFinancialLiabilities
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The cash outflow to settle borrowings, classified as financing activities. [Refer: Borrowings]

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 7
-IssueDate 2019-01-01
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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=gsm_FinancialLoansWithGovernmentsAgenciesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=gsm_LoanNumber1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=gsm_LoanNumber2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=ifrs-full_DerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	gsm_IfrsBalanceSheetLocationAxis=gsm_CurrentLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	gsm_IfrsBalanceSheetLocationAxis=gsm_NoncurrentLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
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