| Document and Entity Information | Jun. 17, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | FIBROGEN INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000921299 |
| Document Type | 8-K |
| Document Period End Date | Jun. 17, 2021 |
| Entity Address, Address Line Two | Inc. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-36740 |
| Entity Tax Identification Number | 77-0357827 |
| Entity Address, Address Line One | FibroGen, Inc. |
| Entity Address, Address Line Three | 409 Illinois Street |
| Entity Address, City or Town | San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94158 |
| City Area Code | (415) |
| Local Phone Number | 978-1200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.01 par value |
| Trading Symbol | FGEN |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
