Condensed Consolidated Statements of Operations (Unaudited) (Parenthetical) - Astellas Agreement [Member] - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
License and milestone revenue from a related party	$ 0	$ 0	$ 0	$ 22,590
Collaboration services and other revenue from a related party	2,358	1,414	5,821	8,419
Product revenue from a related party	$ 26,463	$ 14,914	$ 68,347	$ 50,873

Revenue from multiple-deliverable arrangements that include licensing fees and services revenue and the amount of consideration recognized during the period for milestones. Licensing revenue is consideration received from another party for the right to use, but not own, certain of the entity's intangible assets. Licensing arrangements include, but are not limited to, rights to use a patent, copyright, technology, manufacturing process, software or trademark. Licensing fees are generally, but not always, fixed as to amount and not dependent upon the revenue generated by the licensing party. An entity may receive licensing fees for licenses that also generate royalty payments to the entity. Services revenue may be derived by providing other, non-specified, services during the reporting period.

No definition available.

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