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Fair Value Measurements - Additional Information (Detail) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2022
Fair Value Disclosures [Abstract] |  |  |
Transfers of assets from level 1 to 2 | $ 0 | $ 0 | $ 10,500,000
Transfers of assets from level 2 to 1 | 0 | 0 |
Transfers of assets into level 3 | 0 | 0 |
Transfers of assets out of level 3 | $ 0 | $ 0 |

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- Definition Fair value assets level1 to level2 transfer amount. + References No definition available. + Details
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- Definition Fair value assets level2 to level1 transfer amount. + References No definition available. + Details
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Namespace Prefix: | fgen_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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- Definition Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258 + Details
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- Definition Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258 + Details
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