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Leases - Additional Information (Detail) - Lease | 12 Months Ended
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Dec. 31, 2020 | 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 | 3 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 |  |
Office Spaces [Member] |  |  |
Lessee Lease Description [Line Items] |  |  |
Number of operating leases | 6 |  |
Lessee, operating lease, expiration period | 2023 |  |
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 | 4 years |  |

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- Definition Lessee, finance lease, additional lease expiration period. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Lessee, finance lease, expiration period. + References No definition available. + Details
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Namespace Prefix: | fgen_
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Balance Type: | na
Period Type: | duration

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- Definition Lessee, finance lease, option to extend the additional lease term. + References No definition available. + Details
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Balance Type: | na
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- Definition Lessee, operating lease, expiration period. + References No definition available. + Details
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Namespace Prefix: | fgen_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

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- 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

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- Definition Number of operating leases. + References No definition available. + Details
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Namespace Prefix: | fgen_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage increases on each anniversary of rent commencement date. + References No definition available. + Details
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Namespace Prefix: | fgen_
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Balance Type: | na
Period Type: | duration

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- 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=121609121&loc=SL77918673-209980 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:booleanItemType
Balance Type: | na
Period Type: | duration

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- 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 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=121609121&loc=SL77918673-209980 + Details
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- 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=121609121&loc=SL77918673-209980 + Details
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- 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
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- 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 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=121609121&loc=SL77918673-209980 + Details
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Balance Type: | na
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- Details
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- Details
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- Details
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Namespace Prefix: |
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- Details
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |