Collaboration Agreements - Total Arrangement Consideration Allocated to Deliverables along with Associated Deferred Revenue (Detail) $ in Thousands	Sep. 30, 2016 USD ($)
Astellas Agreement [Member] \| Japan [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	$ 47,551
Deferred Revenue	26
Total Consideration	47,577
Astellas Agreement [Member] \| Japan [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	45,403
Deferred Revenue	0
Total Consideration	45,403
Astellas Agreement [Member] \| Japan [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	20
Deferred Revenue	26
Total Consideration	46
Astellas Agreement [Member] \| Japan [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	2,108
Deferred Revenue	0
Total Consideration	2,108
Astellas Agreement [Member] \| Japan [Member] \| Committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	20
Deferred Revenue	0
Total Consideration	20
Astellas Agreement [Member] \| Europe [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	452,224
Deferred Revenue	417
Total Consideration	452,641
Astellas Agreement [Member] \| Europe [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	409,012
Deferred Revenue	0
Total Consideration	409,012
Astellas Agreement [Member] \| Europe [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	362
Deferred Revenue	417
Total Consideration	779
Astellas Agreement [Member] \| Europe [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	9,797
Deferred Revenue	0
Total Consideration	9,797
Astellas Agreement [Member] \| Europe [Member] \| Committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	283
Deferred Revenue	0
Total Consideration	283
Astellas Agreement [Member] \| Europe [Member] \| Development services-in progress [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	32,770
Deferred Revenue	0
Total Consideration	32,770
AstraZeneca Agreements [Member] \| U.S./RoW [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	465,959
Deferred Revenue	112,150
Total Consideration	578,109
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	382,842
Deferred Revenue	0
Total Consideration	382,842
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	358
Deferred Revenue	40
Total Consideration	398
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Co-development, information sharing & committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	82,759
Deferred Revenue	33,638
Total Consideration	116,397
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	78,472
Total Consideration	$ 78,472

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.3)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

Reference 2: 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)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

+ Details

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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_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
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- Details

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

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

Name:	us-gaap_ProductOrServiceAxis=fgen_DevelopmentServicesInProgressMember
Namespace Prefix:
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- Details

Name:	us-gaap_TypeOfArrangementAxis=fgen_AstraZenecaAgreementsMember
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- Details

Name:	us-gaap_StatementGeographicalAxis=fgen_UnitedStatesAndRestOfWorldMember
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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_CoDevelopmentInformationSharingAndCommitteeServicesMember
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