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Management of Financial Risk (Disclosure of Nature and Extent Arising from Financial Instruments - Commodity Price Risk) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Jan. 26, 2018
Silver Commodity Type [member] | Commodity price risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 3,424 |
Change percentage, commodity | 10.00% |
Gold Commodity Type [member] | Commodity price risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 1,767 |
Change percentage, commodity | 10.00% |
Lead Commodity [member] | Commodity price risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 216 |
Change percentage, commodity | 10.00% |
Zinc Commodity [member] | Commodity price risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Effect on Sales | $ 281 |
Change percentage, commodity | 10.00% |
Non-Revolving Credit Facility [Member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Borrowing capcaity |  | $ 40,000
Non-Revolving Credit Facility [Member] | Interest rate risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Borrowing capcaity | $ 40,000 |

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References No definition available. + Details
Name: | fsm_IfrsLineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | fsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Percentage change on commodity price percentage. + References No definition available. + Details
Name: | fsm_PercentageChangeOnCommodityPricePercentage
Namespace Prefix: | fsm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage price change on commodity effect on sales amount. + References No definition available. + Details
Name: | fsm_PercentagePriceChangeEffectOnSalesAmount
Namespace Prefix: | fsm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
Name: | ifrs-full_DisclosureOfNatureAndExtentOfRisksArisingFromFinancialInstrumentsLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | fsm_TypeOfCommodityAxis=fsm_SilverCommodityTypeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_TypesOfRisksAxis=ifrs-full_CommodityPriceRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | fsm_TypeOfCommodityAxis=fsm_GoldCommodityTypeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | fsm_TypeOfCommodityAxis=fsm_LeadCommodityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | fsm_TypeOfCommodityAxis=fsm_ZincCommodityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_BorrowingsByNameAxis=fsm_DebtInstrumentNonRevolvingCreditFacilityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_TypesOfRisksAxis=ifrs-full_InterestRateRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |