Collaboration Agreements - Total Arrangement Consideration Allocated to Deliverables along with Associated Deferred Revenue (Detail) $ in Thousands	Sep. 30, 2017 USD ($)
Astellas Agreement [Member] \| Japan [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	$ 48,855
Deferred Revenue	24
Total Consideration	48,879
Astellas Agreement [Member] \| Japan [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	46,646
Deferred Revenue	0
Total Consideration	46,646
Astellas Agreement [Member] \| Japan [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	24
Deferred Revenue	24
Total Consideration	48
Astellas Agreement [Member] \| Japan [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	2,164
Deferred Revenue	0
Total Consideration	2,164
Astellas Agreement [Member] \| Japan [Member] \| Committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	21
Deferred Revenue	0
Total Consideration	21
Astellas Agreement [Member] \| Europe [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	467,739
Deferred Revenue	394
Total Consideration	468,133
Astellas Agreement [Member] \| Europe [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	422,988
Deferred Revenue	0
Total Consideration	422,988
Astellas Agreement [Member] \| Europe [Member] \| When and if available compounds [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	434
Deferred Revenue	394
Total Consideration	828
Astellas Agreement [Member] \| Europe [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	10,132
Deferred Revenue	0
Total Consideration	10,132
Astellas Agreement [Member] \| Europe [Member] \| Committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	293
Deferred Revenue	0
Total Consideration	293
Astellas Agreement [Member] \| Europe [Member] \| Development services-in progress [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	33,892
Deferred Revenue	0
Total Consideration	33,892
AstraZeneca Agreements [Member] \| U.S./RoW [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	564,103
Deferred Revenue	118,400
Total Consideration	682,503
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| License [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	451,974
Deferred Revenue	0
Total Consideration	451,974
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Manufacturing-clinical supplies [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	436
Deferred Revenue	34
Total Consideration	470
AstraZeneca Agreements [Member] \| U.S./RoW [Member] \| Co-development, information sharing & committee services [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Cumulative Revenue	111,693
Deferred Revenue	25,723
Total Consideration	137,416
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	92,643
Total Consideration	$ 92,643

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

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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

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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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- Details

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- Details

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- Details

Name:	us-gaap_ProductOrServiceAxis=fgen_LicenseMember
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- Details

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

Name:	us-gaap_ProductOrServiceAxis=fgen_ManufacturingClinicalSuppliesMember
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- Details

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- Details

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