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Leases - Schedule of Supplemental Cash Flow Information Related to Leases (Detail) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2022 | Mar. 31, 2021
Cash paid for amounts included in the measurement of lease liabilities: |  |
Financing cash flows from finance leases | $ 1 | $ 3,299

X
- Definition Cash paid for amounts included in the measurement of lease liabilities. + References No definition available. + Details
Name: | fgen_CashPaidForAmountsIncludedInMeasurementOfLeaseLiabilitiesAbstract
Namespace Prefix: | fgen_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Repayments of finance lease liabilities. + References No definition available. + Details
Name: | fgen_RepaymentsOfFinanceLeaseLiabilities
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration