| Document and Entity Information | Aug. 06, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Aug. 6, 2020 |
| Entity Registrant Name | PRINCIPIA BIOPHARMA INC. |
| Entity Central Index Key | 0001510487 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity File Number | 001-38653 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 26-3487603 |
| Entity Address, Address Line One | 220 East Grand Avenue |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 650 |
| Local Phone Number | 416-7700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of each class | Common Stock, Par Value $0.0001 Per Share |
| Trading Symbol(s) | PRNB |
| Name of each exchange on which registered | NASDAQ |
