The amount of additional consideration based on net sales of product as defined under a collaboration agreement.

No definition available.

Name:	fgen_AdditionalConsiderationBasedOnNetSales
Namespace Prefix:	fgen_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of milestone payments related to clinical development under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	fgen_ClinicalDevelopmentMilestone
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the amount of milestone payments related to commercial sales under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	fgen_CommercialSalesMilestone
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the potential amount of milestone payments related to development and regulatory approval under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	fgen_DevelopmentAndRegulatoryApprovalMilestones
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the percentage of costs that will be funded by the other party under a collaborative agreement for development costs.

+ References

No definition available.

+ Details

Name:	fgen_PercentageOfFundingForCosts
Namespace Prefix:	fgen_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- 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

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

Collaboration Agreements - Astellas Agreements - Additional Information (Detail) - Astellas Agreement [Member] - USD ($) $ in Millions	1 Months Ended		3 Months Ended	12 Months Ended			35 Months Ended	45 Months Ended
	Apr. 30, 2006	Jun. 30, 2005	Mar. 31, 2016	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010	Feb. 28, 2009	Feb. 28, 2009
Japan [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront, non-contingent and time-based payments received								$ 40.1
Development and regulatory approval milestones		$ 117.5
Commercial sales milestone		$ 15.0
Additional consideration based on net sales description			Low 20% range
Clinical development milestones				$ 12.5
Europe [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront, non-contingent and time-based payments received							$ 320.0
Development and regulatory approval milestones	$ 425.0
Additional consideration based on net sales description			Low 20% range
Clinical development milestones					$ 50.0	$ 40.0
Percentage of joint development costs committed to fund	50.00%