EARNINGS PER SHARE - Additional Information (Details) $ in Thousands	12 Months Ended
Dec. 31, 2025 USD ($)
Earnings per share [abstract]
Convertible debt, net of tax	$ 0

Finance costs on potentially dilutive convertible debt, tax amount

No definition available.

Name:	fsm_FinanceCostsOnPotentiallyDilutiveConvertibleDebtTaxAmount
Namespace Prefix:	fsm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration