Other liabilities - RSUs and SARs (Details) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Restricted stock units
Disclosure of terms and conditions of share-based payment arrangement [line items]
Vesting life of share-based awards	3 years
Share options exercised	0	7,031
Outstanding	26,268	26,268
Share based compensation expense	$ 17,000	$ 584,000
Share based compensation expense, after tax	11,000	376,000
Current liability from share based payments	$ 26,000	$ 41,000
Stock appreciation rights
Disclosure of terms and conditions of share-based payment arrangement [line items]
Vesting life of share-based awards	3 years
Vesting percentage of share-based awards	33.33%
Cancelled	150,000	74,373
Exercised	0	498,476
Outstanding	460,021	610,021
Share based compensation expense	$ 61,000	$ 5,848,000
Share based compensation expense, after tax	39,000	3,762,000
Current liability from share based payments	$ 2,000	$ 62,000

The amount of expense from share-based payment transactions with employees after tax.

No definition available.

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

- Definition

The amount of current liabilities arising from share-based payment transactions. Share-based payment transactions are transactions in which the entity: (a) receives goods or services from the supplier of those goods or services (including an employee) in a share-based payment arrangement; or (b) incurs an obligation to settle the transaction with the supplier in a share-based payment arrangement when another group entity receives those goods or services.

+ References

No definition available.

+ Details

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

- Definition

The number of other equity instruments (ie other than share options) cancelled in a share-based payment arrangement.

+ References

No definition available.

+ Details

Name:	gsm_NumberOfOtherEquityInstrumentsCancelledInShareBasedPaymentArrangement
Namespace Prefix:	gsm_
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Balance Type:	na
Period Type:	duration

- Definition

The vesting life of awards in a share-based payment arrangement.

+ References

No definition available.

+ Details

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

- Definition

The vesting percentage of awards in share-based payment arrangement.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

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

- Definition

The amount of expense from share-based payment transactions with employees.

+ References

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
-URIDate 2019-03-27

+ Details

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

- Definition

The number of other equity instruments (ie other than share options) exercised or vested in a share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 2
-IssueDate 2019-01-01
-Paragraph 45
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_45&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The number of other equity instruments (ie other than share options) outstanding in a share-based payment arrangement.

+ References

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of share options exercised in a share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2019-01-01
-Paragraph 45
-Subparagraph b
-Clause iv
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2019-en-r&anchor=para_45_b_iv&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
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- Details

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