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Equity and Stock-based Compensation - Warrants - Additional information (Detail) - shares | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2020
Shareholders Equity And Stock Based Compensation [Abstract] |  |
Warrants to purchase common stock exercised | 4,430 |
Warrants to purchase common stock outstanding | 0 | 0

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 No definition available. + Details
Name: | us-gaap_ClassOfWarrantOrRightOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant