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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

X

- Definition

Fair value asset level 1 to level 2 transfers amount.

+ References

No definition available.

+ Details

Name:	fgen_FairValueAssetLevel1ToLevel2TransfersAmount
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_FairValueDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration