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
Reverse of prior year tax charge	$ 1.7

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.

No definition available.

Name:	gsm_IfrsTaxCutsAndJobsActOf2017IncompleteAccountingTransitionTaxForAccumulatedForeignEarningsProvisionalIncomeTaxExpense
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration