Financial assets and other receivables - Government Grants (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Financial assets and other receivables
Indirect carbon dioxide emission compensation deducted against the related expense in "Raw Materials and energy consumption for production"	$ 75,637	$ 82,553

The amount of indirect carbon dioxide emission compensation deducted against the related expenses in "Raw Materials and energy consumption for production" in the consolidated income statement.

No definition available.

Name:	gsm_IndirectCarbonDioxideEmissionCompensationDeductedAgainstRelatedExpenses
Namespace Prefix:	gsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration