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Liability Related to Sale of Future Royalties - Schedule of Activity of Liability Related to Sale of Future Revenues (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Liability Related to Sale of Future Royalties [Abstract] |  |  |
Liability related to sale of future revenues - beginning balance | $ 0 |  |
Proceeds from sale of future revenues, gross | 50,000 |  |
Less: Initial issuance costs | (250) |  |
Less: Transaction costs | (1,453) |  |
Interest expense recognized | 1,036 |  |
Liability related to sale of future revenues - ending balance | 49,333 | $ 0 |
Less: Current portion classified within accrued and other current liabilities | 0 |  |
Liability related to sale of future revenues, non-current | $ 49,333 |  |

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- Definition Gross proceeds from sale of future revenues. + References No definition available. + Details
Name: | fgen_GrossProceedsFromSaleOfFutureRevenues
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Initial issuance costs related to sale of future revenues. + References No definition available. + Details
Name: | fgen_InitialIssuanceCostsRelatedToSaleOfFutureRevenues
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- Definition Transaction costs related to sale of future revenues. + References No definition available. + Details
Name: | fgen_TransactionCostsRelatedToSaleOfFutureRevenues
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration