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Liability Related to Sale of Future Revenues - Schedule of Activities of Liability Related to Sale of Future Revenues (Detail) - USD ($) $ in Thousands | 9 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024
Liability Related to Sale of Future Royalties [Abstract] |  |
Liability related to sale of future revenues - beginning balance | $ 59,324 |
Interest paid | (450) | $ (5,700)
Interest expense recognized | 6,109 |
Liability related to sale of future revenues - ending balance | 64,983 |
Less: Current portion classified to accrued and other current liabilities | (1,569) |
Liability related to sale of future revenues, non-current | $ 63,414 |

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- Definition Interest paid related to sale of future revenues. + References No definition available. + Details
Name: | fgen_InterestPaidRelatedToSaleOfFutureRevenues
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Liability related to sale of future revenues. + References No definition available. + Details
Name: | fgen_LiabilityRelatedToSaleOfFutureRevenues
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Liability related to sale of future royalties. + References No definition available. + Details
Name: | fgen_LiabilityRelatedToSaleOfFutureRoyaltiesAbstract
Namespace Prefix: | fgen_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Liability related to sale of revenue current. + References No definition available. + Details
Name: | fgen_LiabilityRelatedToSaleOfRevenueCurrent
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Liability related to sale of revenues noncurrent. + References No definition available. + Details
Name: | fgen_LiabilityRelatedToSaleOfRevenuesNoncurrent
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Non cash interest expense related to sale of future revenues. + References No definition available. + Details
Name: | fgen_NonCashInterestExpenseRelatedToSaleOfFutureRevenues
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration