Collaboration Agreements,License Agreement and Revenues - Roll-forward of Related Contract Liabilities (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2021 USD ($)
Contract with Customer Liability [Line Items]
Balance at December 31, 2020	$ (2,147)
Balance at December 31, 2021	(58,909)
AstraZeneca Agreements [Member]
Contract with Customer Liability [Line Items]
Balance at December 31, 2020	(137,338)
Additions	(69,746)
Balance at December 31, 2021	(171,516)
Milestone payment received	35,568
Product [Member] \| Direct Sales [Member] \| Contract Liabilities [Member]
Contract with Customer Liability [Line Items]
Balance at December 31, 2020	(15,137)
Additions	(1,371)
Deduction	13,645
Currency Translation and Other	(313)
Balance at December 31, 2021	(3,176)
Drug Product Revenue [Member]
Contract with Customer Liability [Line Items]
Balance at December 31, 2020	(5,984)
Additions	(83,019)
Balance at December 31, 2021	(39,036)
Milestone payment received	179
Balance Presented Net Against Contract Asset	49,788
Drug Product Revenue [Member] \| AstraZeneca Agreements [Member] \| U.S. [Member]
Contract with Customer Liability [Line Items]
Balance at December 31, 2020	0
Additions	(11,171)
Balance at December 31, 2021	(11,171)
Milestone payment received	0
Balance Presented Net Against Contract Asset	0
Drug Product Revenue [Member] \| Astellas Agreement \| Japan [Member]
Contract with Customer Liability [Line Items]
Balance at December 31, 2020	0
Additions	(1,974)
Balance at December 31, 2021	(1,974)
Milestone payment received	0
Balance Presented Net Against Contract Asset	0
Drug Product Revenue [Member] \| Astellas Agreement \| Europe [Member]
Contract with Customer Liability [Line Items]
Balance at December 31, 2020	(5,984)
Additions	(69,874)
Balance at December 31, 2021	(25,891)
Milestone payment received	179
Balance Presented Net Against Contract Asset	$ 49,788

Contract with customer liability additions.

No definition available.

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

- Definition

Contract with customer liability deduction.

+ References

No definition available.

+ Details

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

- Definition

Contract with customer, liability, increase (decrease) from current translation and other.

+ References

No definition available.

+ Details

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

- Definition

Contract with customer liability.

+ References

No definition available.

+ Details

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

- Definition

Contract with customer liability net of balance presented against contract asset.

+ References

No definition available.

+ Details

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

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=123360276&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=123360276&loc=SL49130532-203044

+ Details

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

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=fgen_AstraZenecaAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=us-gaap_ProductMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=fgen_DirectSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=fgen_ContractLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=fgen_DrugProductRevenueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=fgen_AstellasAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_JP
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=srt_EuropeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: