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

No definition available.

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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

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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

Estimated joint development extended service period.

+ References

No definition available.

+ Details

Name:	fgen_EstimatedJointDevelopmentExtendedServicePeriod
Namespace Prefix:	fgen_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- 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

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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

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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

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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 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=118956577&loc=SL49130556-203045

+ Details

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

- Definition

Amount of consideration paid in advance for supplies that provide economic benefits within a future period of one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (g)(7)
-URI http://asc.fasb.org/extlink&oid=118951113&loc=d3e6676-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118951113&loc=d3e6787-107765

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- 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
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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
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Period Type:

Collaboration Agreements - Astellas Agreements - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended			9 Months Ended	12 Months Ended	35 Months Ended	45 Months Ended
	Sep. 30, 2019	Apr. 30, 2006	Jun. 30, 2005	Sep. 30, 2019	Dec. 31, 2018	Feb. 28, 2009	Feb. 28, 2009
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Unbilled contract asset	$ 180,000			$ 180,000	$ 0
Europe [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Unbilled contract asset	130,000			130,000
Astellas Agreement [Member] \| Japan [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront, non-contingent and time-based payments received							$ 40,100
Potential milestone payments	12,500		$ 132,500
Commercial sales milestone			15,000
Aggregate consideration received	77,600			77,600
Transaction price and allocated to performance obligations	12,500			12,500
Astellas Agreement [Member] \| Japan [Member] \| Clinical and Development Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments			22,500
Astellas Agreement [Member] \| Japan [Member] \| Regulatory Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments			$ 95,000
Astellas Agreement [Member] \| Europe [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront, non-contingent and time-based payments received						$ 320,000
Potential milestone payments		$ 425,000
Aggregate consideration received	410,000			410,000
Transaction price and allocated to performance obligations	130,000			$ 130,000
Percentage of joint development costs committed to fund		50.00%
Additional consideration based on net sales description				low 20% range
Estimated joint development extended service period					2023
Revenue during period from performance obligations	129,000			$ 129,000
Unbilled contract asset	$ 130,000			$ 130,000
Astellas Agreement [Member] \| Europe [Member] \| Clinical and Development Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments		$ 90,000
Astellas Agreement [Member] \| Europe [Member] \| Regulatory Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential milestone payments		$ 335,000