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v3.25.3

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2025
Fair Value Disclosures [Abstract] |  |
Transfers of assets from level 1 to 2 | $ 10.0 | $ 17.8

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- Definition Fair value asset level 1 to level 2 transfers amount. + 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
Name: | us-gaap_FairValueDisclosuresAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration