Effective Annual Interest Rate Related To Sale Of Future Revenues

No definition available.

Name:	fgen_EffectiveAnnualInterestRateRelatedToSaleOfFutureRevenues
Namespace Prefix:	fgen_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Future revenue granted.

+ References

No definition available.

+ Details

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

- Definition

Interest paid related to sale of future revenues.

+ References

No definition available.

+ Details

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

- Definition

Maximum payment cap amount.

+ References

No definition available.

+ Details

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

- Definition

Non-cash interest expense related to drug product revenue

+ References

No definition available.

+ Details

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

- Definition

Non cash interest expense related to sale of future revenues.

+ References

No definition available.

+ Details

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

- Definition

Percentage of revenue interest on global net sales.

+ References

No definition available.

+ Details

Name:	fgen_PercentageOfRevenueInterestOnGlobalNetSales
Namespace Prefix:	fgen_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of revenue sold.

+ References

No definition available.

+ Details

Name:	fgen_PercentageOfRevenueSold
Namespace Prefix:	fgen_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Prepayment amount.

+ References

No definition available.

+ Details

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

- Definition

Revenue interest payment.

+ References

No definition available.

+ Details

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

- Definition

Revenue payment cap amount.

+ References

No definition available.

+ Details

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 808
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479402/808-10-50-1

+ Details

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

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-41

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 270
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482964/270-10-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (ee)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 4: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 5: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-32

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 924
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479941/924-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-5

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-30

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-40

Reference 12: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-22

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-4

+ Details

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

- Details

Name:	srt_CounterpartyNameAxis=fgen_NovaquestCapitalManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=fgen_RevenueInterestFinancingAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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_AwardDateAxis=fgen_PaymentPeriodOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fgen_PaymentCapDateOnOrBeforeDecemberThirtyOneTwoThousandTwentyEightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fgen_PaymentCapDateOnOrAfterJanuaryOneTwoThousandTwentyNineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fgen_PaymentCapDateAfterJanuaryOneTwoThousandThirtyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fgen_PaymentPeriodTwoMember
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:	srt_ProductOrServiceAxis=us-gaap_LicenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=fgen_DevelopmentAndOtherRevenueMember
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_ProductOrServiceAxis=fgen_MilestonePaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=fgen_DevelopmentRevenueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationAllowancesAndReservesTypeAxis=fgen_DrugProductRevenueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Liability Related to Sale of Future Revenues - Additional Information (Detail) - USD ($)		3 Months Ended			9 Months Ended
	Nov. 04, 2022	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total revenue		$ 46,333,000		$ 40,134,000	$ 152,877,000	$ 120,614,000
Interest expense related to sale of future revenues					5,768,000
Interest paid			$ (5,700,000)		$ (5,653,000)
NovaQuest Capital Management [Member] \| Revenue Interest Financing Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Effective annual interest rate related to sale of future revenues					15.59%
NovaQuest Capital Management [Member] \| Revenue Interest Financing Agreement [Member] \| Minimum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Prepayment amount of 2024 to 2030	$ 80,000,000
NovaQuest Capital Management [Member] \| Revenue Interest Financing Agreement [Member] \| Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Prepayment amount of 2024 to 2030	125,000,000
NovaQuest Capital Management [Member] \| Payment Period One [Member] \| Revenue Interest Financing Agreement [Member] \| Minimum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue interest payment	62,500,000
NovaQuest Capital Management [Member] \| Payment Cap Date on or before December 31, 2028 [Member] \| Revenue Interest Financing Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum payment cap amount	100,000,000
NovaQuest Capital Management [Member] \| Payment Cap Date on or after January 1, 2029 [Member] \| Revenue Interest Financing Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum payment cap amount	112,500,000
NovaQuest Capital Management [Member] \| Payment Cap Date after January 1, 2030 [Member] \| Revenue Interest Financing Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum payment cap amount	125,000,000
NovaQuest Capital Management [Member] \| Payment Period Two [Member] \| Revenue Interest Financing Agreement [Member] \| Minimum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue interest payment	125,000,000
Product [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total revenue		46,210,000		29,390,000	$ 126,391,000	77,439,000
Product [Member] \| NovaQuest Capital Management [Member] \| Revenue Interest Financing Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue interest payment	10,000,000
License [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total revenue		0		2,649,000	0	9,649,000
Other Revenue - Contract Manufacturing [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total revenue		385,000		6,775,000	1,532,000	15,825,000
Astellas Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Future revenue granted	$ 50,000,000
Percentage of revenue interest on global net sales	100.00%
Astellas Agreement [Member] \| Maximum [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue payment cap amount		125,000,000			125,000,000
Astellas Agreement [Member] \| Product [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of revenue sold	22.50%
Astellas Agreement [Member] \| Milestone payments [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of revenue sold	10.00%
Astellas Agreement [Member] \| Milestone payments [Member] \| Payment Period One [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of revenue sold	20.00%
Development Revenue [Member] \| Astellas Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total revenue		400,000			1,000,000
Drug Product Revenue [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total revenue		(262,000)		1,320,000	24,954,000	17,701,000
Drug Product Revenue [Member] \| Astellas Agreement [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total revenue		300,000			700,000
Non-cash interest expense related to drug product revenue		$ 2,000,000		$ 2,100,000	$ 5,800,000	$ 5,600,000