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Tax matters - Tax Cuts and Jobs Act (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Tax matters
Applicable tax rate	21.00%	35.00%
Deferred tax credit	$ 31.2
One off tax charge	$ 1.7

X

- Definition

Amount of reasonable estimate for income tax benefit for remeasurement of deferred tax liability from change in tax rate pursuant to Tax Cuts and Jobs Act of 2017 for which accounting for tax effect is incomplete.

+ References

No definition available.

+ Details

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X

- Definition

Amount of reasonable estimate for income tax expense from transition tax on accumulated earnings of controlled foreign corporation deemed repatriated pursuant to Tax Cuts and Jobs Act of 2017 for which accounting for tax effect is incomplete.

+ References

No definition available.

+ Details

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

X

- Definition

The applicable income tax rate.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2018-01-01
-Paragraph 81
-Subparagraph c
-Clause ii
-URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=12&date=2018-03-01&anchor=para_81_c_ii&doctype=Standard
-URIDate 2018-03-16

+ Details

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Namespace Prefix:	ifrs-full_
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Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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