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

X

- Definition

Fair value assets level1 to level2 transfer amount.

+ References

No definition available.

+ Details

Name:	fgen_FairValueAssetsLevel1ToLevel2TransferAmount
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