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Condensed Consolidated Statements of Operations (Unaudited) (Parenthetical) - Astellas Agreement [Member] - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Collaboration services and other revenue from a related party | $ 3,611 | $ 4,737
Product revenue from a related party | 10,406 | 0
Drug product revenue from a related party | $ 4,030 | $ 0

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- Definition Revenue recognized from co-development services, manufacturing of clinical supplies, committee services and information services and other revenue. + References No definition available. + Details
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Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Drug product revenue from related party. + References No definition available. + Details
Name: | fgen_DrugProductRevenueFromRelatedParty
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Product revenue from a related party. + References No definition available. + Details
Name: | fgen_ProductRevenue
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=fgen_AstellasCollaborationAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |