Condensed Consolidated Statements of Operations (Unaudited) (Parenthetical) - Astellas Agreement [Member] - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Collaboration services and other revenue from a related party	$ 2,645	$ 4,766	$ 6,256	$ 9,503
Product revenue from a related party	11,760	0	22,167	0
Drug product revenue from a related party	$ (1,974)	$ 8,238	$ 2,056	$ 8,238

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