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

No definition available.

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

- Definition

Represents the amount of contingent payments provided for under the collaboration agreement.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

The non-contingent performance period related to a deliverable under a collaboration agreement.

+ References

No definition available.

+ Details

Name:	fgen_NonContingentPerformancePeriod
Namespace Prefix:	fgen_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the total amount of non-substantive milestones that could be earned upon the achievement of certain pre-determined events from the total potential amount of milestone payments under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

Name:	fgen_PotentialMilestones
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of upfront payments included under the collaborative agreement as arrangement consideration with extended terms.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of non-contingent upfront payments received under the collaborative agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of remaining upfront and non-contingent and time based payments under the collaboration agreement.

+ References

No definition available.

+ Details

Name:	fgen_RemainingUpfrontNonContingentAndTimeBasedPayments
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Implied royalty rate used to find the net present value of future cash flows.

+ References

No definition available.

+ Details

Name:	fgen_RoyaltyRateOnNetRevenue
Namespace Prefix:	fgen_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the total amount of substantive milestones that could be earned upon the achievement of certain pre-determined events from the total potential amount of milestone payments under the collaborative agreement.

+ References

No definition available.

+ Details

Name:	fgen_SubstantiveMilestones
Namespace Prefix:	fgen_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=fgen_ClinicalDevelopmentMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=fgen_RegulatoryMilestonesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Collaboration Agreements - Accounting for the AstraZeneca Agreements - Additional Information 3 (Detail) - AstraZeneca Agreements [Member] - USD ($) $ in Millions			6 Months Ended	9 Months Ended	12 Months Ended	30 Months Ended
	Dec. 31, 2013	Jul. 30, 2013	Dec. 31, 2013	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Royalty rate against projected net revenues					40.00%	40.00%
Discount rate applied					17.50%
U.S./RoW [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Non-contingent performance period		65 months
Expected upfront, non-contingent and time-based payments		$ 374.0
Upfront, non-contingent and time-based payments received			$ 82.0			$ 312.0
Remaining up-front, non-contingent and time-based payments		292.0
Upfront, non-contingent and time-based payments with extended payment terms		230.0
Contingent payment		62.0
Total potential milestones		875.0
Non-substantive milestones		160.0
Commercial sales milestone		325.0
U.S./RoW [Member] \| Clinical and Development Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Substantive milestones		65.0
U.S./RoW [Member] \| Regulatory Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Substantive milestones		325.0
China [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront, non-contingent and time-based payments received	$ 16.2
Upfront, non-contingent and time-based payments with extended payment terms		12.0
Contingent payment		20.0
Total potential milestones		328.5
Commercial sales milestone		167.5
Proceeds from upfront payments		28.2
Upfront payments received				$ 12.0
China [Member] \| Clinical and Development Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Substantive milestones		15.0
China [Member] \| Regulatory Milestone [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Substantive milestones		$ 146.0