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SALES (Mitigating Price Risk) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022
SALES [abstract] |  |
Realized loss | $ (1.5) | $ (0.7)

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- Definition The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings. + References No definition available. + Details
Name: | fsm_IfrsGainLossOnSaleOfDerivatives
Namespace Prefix: | fsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | ifrs-full_RevenueAbstract
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration