Acquisition and variable interest entity line items.

No definition available.

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

- Definition

Equity method investment increase (decrease) from currency translation.

+ References

No definition available.

+ Details

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

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 323
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481687/323-10-50-3

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	instant

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 323
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-Section 45
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
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-Publisher FASB
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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of income (loss) for proportionate share of equity method investee's income (loss).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
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+ Details

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

- Definition

Amount of dividend income on nonoperating securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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+ Details

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Balance Type:	credit
Period Type:	duration

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=fgen_BeijingFalikangPharmaceuticalCoLtdMember
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Equity method investment - Variable Interest Entity - Summary of Equity Method Investment (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Acquisition And Variable Interest Entity [Line Items]
Beginning balance			$ 5,061
Share of Net Income	$ 677	$ 407	2,023	$ 1,293
Ending balance	$ 4,534		$ 4,534
Beijing Falikang Pharmaceutical Co. Ltd [Member]
Acquisition And Variable Interest Entity [Line Items]
Ownership Percentage	51.10%		51.10%
Beginning balance			$ 5,061
Share of Net Income			2,023
Dividend Received			(2,255)
Currency Translation			(295)
Ending balance	$ 4,534		$ 4,534