The amount of additional consideration based on net sales of product as defined under a collaboration agreement.

No definition available.

Name:	fgen_AdditionalConsiderationBasedOnNetSales
Namespace Prefix:	fgen_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of aggregate consideration receivable under collaborative arrangement.

+ References

No definition available.

+ Details

Name:	fgen_CollaborativeArrangementAggregateConsideration
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of milestone payments related to commercial sales under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	fgen_CommercialSalesMilestone
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the potential amount of milestone payments related to development and regulatory approval under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	fgen_DevelopmentAndRegulatoryApprovalMilestones
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the percentage of costs that will be funded by the other party under a collaborative agreement for development costs.

+ References

No definition available.

+ Details

Name:	fgen_PercentageOfFundingForCosts
Namespace Prefix:	fgen_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the total potential amount of milestone payments related to development, regulatory approval and commercial sales under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	fgen_PotentialMilestones
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the amount of fixed and determinable non-contingent upfront payments included under the collaborative agreement as arrangement consideration.

+ References

No definition available.

+ Details

Name:	fgen_ProceedsFromUpfrontNonContingentAndTimeBasedPayments
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Transaction price allocated to performance obligations.

+ References

No definition available.

+ Details

Name:	fgen_TransactionPriceAllocatedToPerformanceObligations
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130549-203045

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130556-203045

+ Details

Name:	us-gaap_RevenueRemainingPerformanceObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_TypeOfArrangementAxis=fgen_AstellasCollaborationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_JP
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=fgen_ClinicalDevelopmentMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=fgen_RegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=srt_EuropeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration Agreements and Revenues - Astellas Agreements - Additional Information (Detail) - Astellas Agreement [Member] - USD ($) $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended	35 Months Ended	45 Months Ended
	Apr. 30, 2006	Jun. 30, 2005	Jun. 30, 2020	Sep. 30, 2020	Feb. 28, 2009	Feb. 28, 2009	Dec. 31, 2019	Jun. 30, 2019
Japan [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront, non-contingent and time-based payments received						$ 40.1
Potential milestone payments		$ 132.5
Commercial sales milestone		15.0
Additional consideration based on net sales description				low 20% range
Aggregate consideration received				$ 90.1
Japan [Member] \| Clinical and Development Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments		22.5
Japan [Member] \| Regulatory Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments		$ 95.0
Europe [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront, non-contingent and time-based payments received					$ 320.0
Additional consideration based on net sales description				low 20% range
Aggregate consideration received				$ 540.0
Development and regulatory approval milestones	$ 425.0
Percentage of joint development costs committed to fund	50.00%
Transaction price and allocated to performance obligations								$ 130.0
Revenue during period from performance obligations				$ 0.6			$ 128.8
Milestone payment received			$ 130.0
Europe [Member] \| Clinical and Development Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments	$ 90.0
Europe [Member] \| Regulatory Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments	$ 335.0