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Fair Value Measurements - Additional Information (Detail) $ in Millions | Mar. 31, 2023 USD ($)
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Fair Value Disclosures [Abstract] |
Fair value of US treasury notes and bills transfered from level 1 to level 2 | $ 7.0

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- Definition Fair value of US treasury notes and bills transfered from level one to level two. + References No definition available. + Details
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Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_FairValueDisclosuresAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration