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Collaboration Agreements - Summary of Revenue Recognized Under the Collaboration Agreements - Additional Information 4 (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Japan [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remainder of transaction price, variable consideration from estimated future co-development billing	$ 2.8
Europe [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remainder of transaction price, variable consideration from estimated future co-development billing	27.4
U.S./RoW and China [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Remainder of transaction price, variable consideration from estimated future co-development billing	$ 284.1

X

- Definition

Variable consideration payment received from estimated future co-development billing.

+ References

No definition available.

+ Details

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

X

- 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

X

- Details

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

X

- Details

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

X

- Details

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