Lessee, finance lease, additional lease expiration period.

No definition available.

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

- Definition

Lessee, finance lease, expiration period.

+ References

No definition available.

+ Details

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

- Definition

Lessee, finance lease, option to extend the additional lease term.

+ References

No definition available.

+ Details

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

- Definition

Number of finance leases.

+ References

No definition available.

+ Details

Name:	fgen_NumberOfFinanceLeases
Namespace Prefix:	fgen_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of operating leases.

+ References

No definition available.

+ Details

Name:	fgen_NumberOfOperatingLeases
Namespace Prefix:	fgen_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage increases on each anniversary of rent commencement date.

+ References

No definition available.

+ Details

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

- Definition

Indicates (true false) whether lessee has option to extend finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeFinanceLeaseExistenceOfOptionToExtend
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of terms and conditions of option to extend lessee's finance lease. Includes, but is not limited to, information about option recognized as part of right-of-use asset and lease liability.

+ References

+ Details

Name:	us-gaap_LesseeFinanceLeaseOptionToExtend
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Term of lessee's finance lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeFinanceLeaseTermOfContract1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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_LesseeLeaseDescriptionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fgen_ShorensteinPropertiesLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fgen_BeijingEconomicTechnologicalDevelopmentAreaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_OfficeEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Leases - Additional Information (Detail) - Lease	9 Months Ended	12 Months Ended
Leases - Additional Information (Detail) - Lease	Sep. 30, 2019	Dec. 31, 2013	Dec. 31, 2006
Minimum [Member]
Lessee Lease Description [Line Items]
Lessee, operating lease, lease term	2 years
Maximum [Member]
Lessee Lease Description [Line Items]
Lessee, operating lease, lease term	4 years
Shorenstein Properties LLC [Member]
Lessee Lease Description [Line Items]
Lessee, finance lease, initial lease term			15 years
Lessee, finance lease, expiration period			2023
Lessee, finance lease, option to extend the additional lease term			10 years
Lessee, finance lease, additional lease expiration period			2033
Percentage increases on each anniversary of rent commencement date			2.00%
Lessee, finance lease, existence of option to extend			true
Lessee, finance lease, option to extend	The Company has an option to extend the lease for an additional 10 years through 2033.
Beijing Economic-Technological Development Area [Member]
Lessee Lease Description [Line Items]
Lessee, finance lease, initial lease term		8 years
Lessee, finance lease, expiration period		2021
Lessee, finance lease, existence of option to extend		false
Building [Member]
Lessee Lease Description [Line Items]
Number of finance leases	2
Number of operating leases	6
Office Equipment [Member] \| Minimum [Member]
Lessee Lease Description [Line Items]
Lessee, operating lease, lease term	2 years
Office Equipment [Member] \| Maximum [Member]
Lessee Lease Description [Line Items]
Lessee, operating lease, lease term	5 years