Collaboration Agreements - Total Arrangement Consideration Allocated to Deliverables along with Associated Deferred Revenue (Detail) $ in Thousands	Jun. 30, 2017 USD ($)
Astellas Agreement [Member] \| Japan [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	$ 48,388
Deferred Revenue	25
Total Consideration	48,413
Astellas Agreement [Member] \| Japan [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	46,201
Deferred Revenue	0
Total Consideration	46,201
Astellas Agreement [Member] \| Japan [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	23
Deferred Revenue	25
Total Consideration	48
Astellas Agreement [Member] \| Japan [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	2,144
Deferred Revenue	0
Total Consideration	2,144
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	463,636
Deferred Revenue	399
Total Consideration	464,035
Astellas Agreement [Member] \| Europe [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	419,308
Deferred Revenue	0
Total Consideration	419,308
Astellas Agreement [Member] \| Europe [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	399
Deferred Revenue	399
Total Consideration	798
Astellas Agreement [Member] \| Europe [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	10,043
Deferred Revenue	0
Total Consideration	10,043
Astellas Agreement [Member] \| Europe [Member] \| Committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	290
Deferred Revenue	0
Total Consideration	290
Astellas Agreement [Member] \| Europe [Member] \| Development services-in progress [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	33,596
Deferred Revenue	0
Total Consideration	33,596
AstraZeneca Agreements [Member] \| U.S./RoW [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	541,401
Deferred Revenue	117,134
Total Consideration	658,535
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	436,103
Deferred Revenue	0
Total Consideration	436,103
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	410
Deferred Revenue	43
Total Consideration	453
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Co-development, information sharing & committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	104,888
Deferred Revenue	27,701
Total Consideration	132,589
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	89,390
Total Consideration	$ 89,390

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

Name:	us-gaap_DeferredRevenue
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

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementGeographicalAxis=fgen_UnitedStatesAndRestOfWorldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_CoDevelopmentInformationSharingAndCommitteeServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_ChinaSingleUnitOfAccountingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: