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Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
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Fair Value Disclosures [Abstract] |  |
Transfers of assets from level 1 to 2 | $ 26.3 | $ 20.4

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- Definition Fair value assets level1 to level2 transfer amount. + 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