Condensed Consolidated Statements of Operations (Unaudited) (Parenthetical) - Astellas Agreement [Member] - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Collaboration services and other revenue from a related party	$ 385	$ 2,358	$ 997	$ 5,821
Product revenue from a related party	42,221	26,463	115,339	68,347
Drug product revenue from a related party	$ (262)	$ 1,320	$ (717)	$ 17,701

Revenue recognized from co-development services, manufacturing of clinical supplies, committee services and information services and other revenue.

No definition available.

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