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v3.20.2

Subsequent Event - Additional Information (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Subsequent Event [Line Items] |  |  |
Accrued long-term co-promotional expenses |  | $ 116,230 | $ 53,071
AstraZeneca Agreements [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Accrued long-term co-promotional expenses |  | $ 116,200 | $ 53,100
Scenario Forecast [Member] | AstraZeneca Agreements [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Accrued long-term co-promotional expenses | $ 82,000 |  |

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- Definition Accrued long-term co-promotional liability non current. + References No definition available. + Details
Name: | fgen_AccruedLongTermCoPromotionalLiabilityNonCurrent
Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_TypeOfArrangementAxis=fgen_AstraZenecaAgreementsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |