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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

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