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Equity and Stock-based Compensation - Warrants - Additional information (Detail) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Shareholders Equity And Stock Based Compensation [Abstract]
Warrants to purchase common stock exercised	4,430
Warrants to purchase common stock outstanding	0	4,000

X

- Definition

Class of warrant or right number of warrants or rights exercised during period.

+ References

No definition available.

+ Details

Name:	fgen_ClassOfWarrantOrRightNumberOfWarrantsOrRightsExercisedDuringPeriod
Namespace Prefix:	fgen_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Shareholders equity and stock-based compensation.

+ References

No definition available.

+ Details

Name:	fgen_ShareholdersEquityAndStockBasedCompensationAbstract
Namespace Prefix:	fgen_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of warrants or rights outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant