Percentage of average undrawn availability.

No definition available.

Name:	gsm_AverageUundrawnAvailabilityPercentage
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents formula amount to be calculated for borrowing capacity as percentage of appraised net orderly liquidation value of Eligible inventory minus reserves.

+ References

No definition available.

+ Details

Name:	gsm_FormulaAmountAsPercentageOfAppraisedNetOrderlyLiquidationValueOfEligibleInventoryOverReserves
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents formula amount to be calculated for borrowing capacity as percentage of cost of Eligible Inventory and eligible foreign-in transit inventory.

+ References

No definition available.

+ Details

Name:	gsm_FormulaAmountAsPercentageOfEligibleInventoryAndForeignInTransitInventory
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents formula amount to be calculated for borrowing capacity as percentage of eligible receivable.

+ References

No definition available.

+ Details

Name:	gsm_FormulaAmountAsPercentageOfEligibleReceivables
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents formula amount to be calculated for borrowing capacity subject to inventory amount.

+ References

No definition available.

+ Details

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

- Definition

Represents formula amount to be calculated for borrowing capacity subject to percentage of inventory.

+ References

No definition available.

+ Details

Name:	gsm_FormulaAmountLimitInventoryPercentage
Namespace Prefix:	gsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents formula amount to be calculated for borrowing capacity subject to receivables amount.

+ References

No definition available.

+ Details

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

- Definition

Period of receivable for the formula for borrowing capacity subject to receivables amount.

+ References

No definition available.

+ Details

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

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

No definition available.

+ Details

Name:	gsm_IfrsLineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

No definition available.

+ Details

Name:	gsm_IfrsLineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The maximum outstanding amount of revolving advances and swing loans limit to calculate undrawn availability.

+ References

No definition available.

+ Details

Name:	gsm_MaximumOutstandingAmountOfRevolvingAdvancesAndSwingLoans
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The maximum undrawn availability of outstanding letters of credit.

+ References

No definition available.

+ Details

Name:	gsm_MaximumUndrawnAvailabilityOutstandingLetterOfCredit
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of receivables pledged as security for liabilities.

+ References

No definition available.

+ Details

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

- Definition

Represents repayment term , terms of the facility provide a spring forward provision which requires the ABL Revolver to be repaid on the date which is three (3) months prior to the maturity date of the senior unsecured notes.

+ References

No definition available.

+ Details

Name:	gsm_SpringForwardRepaymentTerm
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

The adjustment to the basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings]

+ References

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

+ Details

Name:	ifrs-full_BorrowingsAdjustmentToInterestRateBasis
Namespace Prefix:	ifrs-full_
Data Type:	num:percentItemType
Balance Type:	na
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 inventories pledged as security for liabilities. [Refer: Inventories]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 2
-IssueDate 2019-01-01
-Paragraph 36
-Subparagraph h
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=2&code=ifrs-tx-2019-en-r&anchor=para_36_h&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The amount of cash and cash equivalents whose use or withdrawal is restricted. [Refer: Cash and cash equivalents]

+ References

+ Details

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

- Details

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

- Details

Name:	gsm_IfrsScenarioAxis=gsm_Level1ScenarioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gsm_IfrsScenarioAxis=gsm_Level3ScenarioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInterestRatesAxis=gsm_FedOvernightBankFundingRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInterestRatesAxis=gsm_IfrsLondonInterbankOfferedRateLiborMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gsm_IfrsScenarioAxis=gsm_Level2ScenarioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfInterestRatesAxis=gsm_DomesticRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_GeographicalAreasAxis=country_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gsm_IfrsInventoryAxis=gsm_InTransitInventoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gsm_IfrsInventoryAxis=gsm_ConsignmentInventoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gsm_IfrsInventoryAxis=gsm_StoresAndSparePartsInventoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gsm_IfrsInventoryAxis=gsm_PackagingMaterialsInventoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Bank borrowings - Credit and Security Agreement - (Details) - USD ($) $ in Thousands	Oct. 11, 2019	Dec. 31, 2019	Dec. 31, 2018
Bank Borrowings
Inventory pledged as security		$ 33,000	$ 314,000
Outstanding balance		158,999	$ 141,012
ABL Revolver
Bank Borrowings
Maximum borrowing capacity	$ 100,000
Credit facility term	5 years
Spring forward repayment term	3 months
Undrawn availability	$ 100,000
Maximum undrawn availability of outstanding letters of credit	10,000
Outstanding amount of revolving advances and swing loans limit	45,000
Restricted cash reserve	22,500
Receivables pledged as security	31,500
Inventory pledged as security	33,000
Outstanding balance	$ 4,500	$ 50,200
ABL Revolver \| Level 1
Bank Borrowings
Percentage of average undrawn availability	66.70%
ABL Revolver \| Level 3
Bank Borrowings
Percentage of average undrawn availability	33.30%
ABL Revolver \| FedOvernightBankFundingRate
Bank Borrowings
Applicable margin (as a percent)	0.50%
ABL Revolver \| LIBOR
Bank Borrowings
Applicable margin (as a percent)	1.00%
ABL Revolver \| LIBOR \| Level 1
Bank Borrowings
Percentage of average undrawn availability	2.50%
ABL Revolver \| LIBOR \| Level 2
Bank Borrowings
Percentage of average undrawn availability	2.75%
ABL Revolver \| LIBOR \| Level 3
Bank Borrowings
Percentage of average undrawn availability	3.00%
ABL Revolver \| Domestic rate \| Level 1
Bank Borrowings
Percentage of average undrawn availability	1.50%
ABL Revolver \| Domestic rate \| Level 2
Bank Borrowings
Percentage of average undrawn availability	1.75%
ABL Revolver \| Domestic rate \| Level 3
Bank Borrowings
Percentage of average undrawn availability	2.00%
ABL Revolver \| Minimum
Bank Borrowings
Formula amount receivable period	90 days
Undrawn availability	$ 10,000
ABL Revolver \| Minimum \| Level 2
Bank Borrowings
Percentage of average undrawn availability	33.30%
ABL Revolver \| Maximum
Bank Borrowings
Formula amount as percentage of eligible receivables	85.00%
Formula amount as percentage of inventory and inventory in transit	75.00%
Formula amount as percentage of inventory and inventory in transit appraised net orderly liquidation value of Eligible inventory over reserves	85.00%
Formula amount subject to percentage of inventory	65.00%
Formula amount receivable period	120 days
Formula amount subject to receivable	$ 5,000
ABL Revolver \| Maximum \| Level 2
Bank Borrowings
Percentage of average undrawn availability	66.70%
ABL Revolver \| CANADA
Bank Borrowings
Undrawn availability	$ 10,000
ABL Revolver \| CANADA \| Maximum
Bank Borrowings
Formula amount subject to inventory	20,000
ABL Revolver \| Eligible in-transit inventory \| Maximum
Bank Borrowings
Formula amount subject to inventory	10,000
ABL Revolver \| Consigned inventory \| Minimum
Bank Borrowings
Formula amount subject to inventory	7,500
ABL Revolver \| Consigned inventory \| Maximum
Bank Borrowings
Formula amount subject to inventory	10,000
ABL Revolver \| Stores and spare parts inventory \| Maximum
Bank Borrowings
Formula amount subject to inventory	2,000
ABL Revolver \| Packaging materials inventory \| Maximum
Bank Borrowings
Formula amount subject to inventory	$ 500