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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

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- 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