Collaboration Agreements - Total Arrangement Consideration Allocated to Deliverables along with Associated Deferred Revenue (Detail) $ in Thousands	Sep. 30, 2015 USD ($)
Astellas Agreement [Member] \| Japan [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	$ 44,119
Deferred Revenue	65
Total Consideration	44,184
Astellas Agreement [Member] \| Japan [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	42,163
Deferred Revenue	0
Total Consideration	42,163
Astellas Agreement [Member] \| Japan [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	14
Deferred Revenue	28
Total Consideration	42
Astellas Agreement [Member] \| Japan [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	1,926
Deferred Revenue	36
Total Consideration	1,962
Astellas Agreement [Member] \| Japan [Member] \| Committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	16
Deferred Revenue	1
Total Consideration	17
Astellas Agreement [Member] \| Europe [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	439,113
Deferred Revenue	643
Total Consideration	439,756
Astellas Agreement [Member] \| Europe [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	397,369
Deferred Revenue	0
Total Consideration	397,369
Astellas Agreement [Member] \| Europe [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	320
Deferred Revenue	436
Total Consideration	756
Astellas Agreement [Member] \| Europe [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	9,318
Deferred Revenue	200
Total Consideration	9,518
Astellas Agreement [Member] \| Europe [Member] \| Committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	268
Deferred Revenue	7
Total Consideration	275
Astellas Agreement [Member] \| Europe [Member] \| Development services-in progress [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	31,838
Deferred Revenue	0
Total Consideration	31,838
AstraZeneca Agreements [Member] \| U.S./RoW [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	320,489
Deferred Revenue	97,993
Total Consideration	418,482
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	277,132
Deferred Revenue	0
Total Consideration	277,132
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	197
Deferred Revenue	91
Total Consideration	288
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Co-development, information sharing & committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	43,160
Deferred Revenue	41,097
Total Consideration	84,257
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| China-single unit of accounting [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	0
Deferred Revenue	56,805
Total Consideration	$ 56,805

The total cash consideration received under a collaboration agreement that has been allocated to the different significant units of accounting and includes the portion that has been deferred for revenue recognition.

No definition available.

+ Details

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

- Definition

Total amount of revenue recognized for license and collaboration services under the collaboration agreement as of a given date.

+ References

No definition available.

+ Details

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

- Definition

Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response)

-URI http://asc.fasb.org/extlink&oid=27012821&loc=d3e214044-122780

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 13

-Section A

+ Details

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Namespace Prefix:	us-gaap_
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

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Namespace Prefix:	us-gaap_
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Balance Type:	na
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- Details

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

Name:	us-gaap_StatementGeographicalAxis=country_JP
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_LicenseMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_WhenAndIfAvailableCompoundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_ManufacturingClinicalSuppliesMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_CommitteeServicesMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_EuropeMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_DevelopmentServicesInProgressMember
Namespace Prefix:
Data Type:	na
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- Details

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

Name:	us-gaap_StatementGeographicalAxis=fgen_UnitedStatesAndRestOfWorldMember
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Data Type:	na
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_CoDevelopmentInformationSharingAndCommitteeServicesMember
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Data Type:	na
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_ChinaSingleUnitOfAccountingMember
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