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FibroGen, Inc. 401(k) Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Defined contribution plan, maximum annual contributions per employee, percent	60.00%
Defined contribution plan, employer matching contributions	$ 3.2	$ 3.0	$ 2.9

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- References

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- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

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