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Management of Financial Risk (Disclosure of Nature and Extent Arising from Financial Instruments - Commodity Price Risk) (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($)
Gold Commodity Type [member] | Commodity price risk [member] |
Disclosure of nature and extent of risks arising from financial instruments [line items] |
Effect on Sales | $ 3,483
Change percentage, commodity | 10.00%
Silver Commodity Type [member] | Commodity price risk [member] |
Disclosure of nature and extent of risks arising from financial instruments [line items] |
Effect on Sales | $ 6,708
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 | $ 279
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 | $ 453
Change percentage, commodity | 10.00%
Lead And Zinc Commodity [member] |
Disclosure of nature and extent of risks arising from financial instruments [line items] |
Percentage of production with forward sales and colloar swaps | 50.00%

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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 of production with forward sales and colloar swaps. + References No definition available. + Details
Name: | fsm_PercentageOfProductionWithForwardSalesAndColloarSwaps
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: | na
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
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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: | ifrs-full_TypesOfRisksAxis=ifrs-full_CommodityPriceRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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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: | 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: | fsm_TypeOfCommodityAxis=fsm_LeadAndZincCommodityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |