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Commitments and Contingencies - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($)
Commitments And Contingencies [Line Items] |
Outstanding non-cancelable contract obligations | $ 86.0
Research and Pre-Clinical Stage Development Programs [Member] |
Commitments And Contingencies [Line Items] |
Maximum future milestone payments | $ 10.9

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- Definition Commitments and contingencies. + References No definition available. + Details
Name: | fgen_CommitmentsAndContingenciesLineItems
Namespace Prefix: | fgen_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum future milestone payments. + References No definition available. + Details
Name: | fgen_MaximumFutureMilestonePayments
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments. + References No definition available. + Details
Name: | us-gaap_ContractualObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=fgen_ResearchAndPreClinicalStageDevelopmentProgramsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |