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INCOME TAX (Narrative) (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
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CANADA |  |
Income Taxes [line items] |  |
Resource related expense accumulated with no deferred tax benefit recognized | $ 7.5 | $ 8.2

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- Definition Resource related expense accumulated with no deferred tax benefit recognized. + References No definition available. + Details
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Namespace Prefix: | fsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |