| Document and Entity Information | Sep. 20, 2019 |
| Document And Entity Information [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 20, 2019 |
| Entity Registrant Name | FIBROGEN INC |
| Entity Central Index Key | 0000921299 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-36740 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 77-0357827 |
| Entity Address, Address Line One | 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 |
| Title of each class | Common Stock, $0.01 par value |
| Trading Symbol | FGEN |
| Name of each exchange on which registered | NASDAQ |
