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Condensed Consolidated Statements of Operations (Unaudited) (Parenthetical) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Astellas Agreement [Member] |  |
Collaboration services and other revenue from a related party | $ 4,859 | $ 5,172

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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
Name: | fgen_CollaborationServicesAndOtherRevenue
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: |