| Document And Entity Information | Feb. 11, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Feb. 11, 2021 |
| Entity Registrant Name | Ultragenyx Pharmaceutical Inc. |
| Entity Central Index Key | 0001515673 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-36276 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 27-2546083 |
| Entity Address, Address Line One | 60 Leveroni Court |
| Entity Address, City or Town | Novato |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94949 |
| City Area Code | 415 |
| Local Phone Number | 483-8800 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value |
| Trading Symbol | RARE |
| Security Exchange Name | NASDAQ |
