Term of lessee's finance lease.

No definition available.

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

- Definition

Time elapsed in lessee's finance lease.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The amount representing the difference between the future minimum finance lease payments and their present value. Minimum lease payments are payments over the lease term that the lessee is or can be required to make, excluding contingent rent, costs for services and taxes to be paid by, and reimbursed to, the lessor, together with: (a) for a lessee, any amounts guaranteed by the lessee or by a party related to the lessee; or (b) for a lessor, any residual value guaranteed to the lessor by: (i) the lessee; (ii) a party related to the lessee; or (iii) a third party unrelated to the lessor that is financially capable of discharging the obligations under the guarantee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2019-01-01
-Name IAS
-Number 17
-IssueDate 2018-01-01
-Paragraph 31
-Subparagraph b
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=17&date=2018-03-02&anchor=para_31_b&doctype=Standard&book=b
-URIDate 2018-03-16

+ Details

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

- Definition

The amount of payments over the finance lease term that the lessee is or can be required to make, excluding contingent rent, costs for services and taxes to be paid by, and reimbursed to, the lessor, together with any amounts guaranteed by the lessee or by a party related to the lessee.

+ References

+ Details

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

- Definition

The amount of assets recognised by the entity, which are leased under finance leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2019-01-01
-Name IAS
-Number 17
-IssueDate 2018-01-01
-Paragraph 31
-Subparagraph a
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=17&date=2018-03-02&anchor=para_31_a&doctype=Standard&book=b
-URIDate 2018-03-16

+ Details

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

- Details

Name:	ifrs-full_ClassesOfAssetsAxis=gsm_HydroelectricalInstallationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Property, plant and equipment - Finance leases (Details) € in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 EUR (€)	Dec. 31, 2017 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Disclosure of recognised finance lease as assets by lessee [abstract]
Interest Payable			$ 8,447	$ 13,331
Lease Payments Outstanding			74,918	95,964
Hydroelectrical Installations
Disclosure of recognised finance lease as assets by lessee [abstract]
Life	10 years	10 years
Time Elapsed	6 years 7 months 6 days	5 years 7 months 6 days
Finance leases			41,918	46,780
Lease Payments Outstanding			65,005	80,639
Hydroelectrical Installations \| Gross carrying amount
Disclosure of recognised finance lease as assets by lessee [abstract]
Finance leases	€ 109,047	€ 109,047	124,859	130,780
Hydroelectrical Installations \| Accumulated depreciation and amortisation
Disclosure of recognised finance lease as assets by lessee [abstract]
Finance leases			$ (82,940)	$ (84,000)