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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

X

- 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

X

- 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

X

- 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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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