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Condensed Consolidated Statements of Operations (Unaudited) (Parenthetical) - Astellas Agreement [Member] - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023
Collaboration services and other revenue from a related party | $ 319 | $ 1,839 | $ 613 | $ 3,463
Product revenue from a related party | 46,005 | 20,512 | 73,118 | 41,884
Drug product revenue from a related party | $ 729 | $ 14,272 | $ (455) | $ 16,381

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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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Data Type: | xbrli:monetaryItemType
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- Definition Drug product revenue from related party. + 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 Product revenue from a related party. + 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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- Details
Name: | us-gaap_TypeOfArrangementAxis=fgen_AstellasCollaborationAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |