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v3.23.3

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2023 | Sep. 30, 2023
Fair Value Disclosures [Abstract] |  |
Fair value of U.S. treasury notes and bills transfered from level 1 to level 2 | $ 8.5 | $ 15.4

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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: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
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