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Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
Commitments And Contingencies [Line Items]
Outstanding non-cancelable contract obligations	$ 86.0
Research and Pre-Clinical Stage Development Programs [Member]
Commitments And Contingencies [Line Items]
Maximum future milestone payments	$ 10.9

X

- Definition

Commitments and contingencies.

+ References

No definition available.

+ Details

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

X

- Definition

Maximum future milestone payments.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_ContractualObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=fgen_ResearchAndPreClinicalStageDevelopmentProgramsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: