Co-development services, estimated continuation year.

No definition available.

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

- Definition

Represents the amount of non-contingent upfront payments included under the collaborative agreement as arrangement consideration.

+ References

No definition available.

+ Details

Name:	fgen_ProceedsFromNonContingentUpfrontPayments
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Provision for co-development services.

+ References

No definition available.

+ Details

Name:	fgen_ProvisionForCoDevelopmentServices
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Variable consideration payment received related to co-development billings.

+ References

No definition available.

+ Details

Name:	fgen_VariableConsiderationPaymentReceivedRelatedToCoDevelopmentBillings
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Variable consideration payment received upon achievement of milestone.

+ References

No definition available.

+ Details

Name:	fgen_VariableConsiderationPaymentReceivedUponAchievementOfMilestone
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Value of input used to measure alternative investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_AlternativeInvestmentMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
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, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

Name:	srt_ProductOrServiceAxis=us-gaap_ManufacturedProductOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_ValuationTechniqueDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Collaboration Agreements - Accounting for the Astellas Agreements - Additional Information 2 (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Manufacturing commercial supplies of products, revenue recognized	$ 29,027,000	$ 40,550,000	$ 104,903,000	$ 100,260,000
Astellas Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Co-development services, currently estimated continuation year			2019
Astellas Agreement [Member] \| Manufacturing Commercial Supplies of Products [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Manufacturing commercial supplies of products, revenue recognized			$ 0
Astellas Agreement [Member] \| Minimum [Member] \| Measurement Input Discount Rate [Member] \| Discounted Cash Flow [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Discount rate applied	17.5		17.5
Astellas Agreement [Member] \| Maximum [Member] \| Measurement Input Discount Rate [Member] \| Discounted Cash Flow [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Discount rate applied	20.0		20.0
Astellas Agreement [Member] \| Japan [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-contingent upfront payments received			$ 40,100,000
Variable consideration related to payments for milestones considered probable of being achieved			37,500,000
Variable consideration related to co-development billings			12,600,000
Provision for co-development services			0
Astellas Agreement [Member] \| Europe [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-contingent upfront payments received			320,000,000
Variable consideration related to payments for milestones considered probable of being achieved			90,000,000
Variable consideration related to co-development billings			$ 183,000,000