| Document and Entity Information | May 06, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | May 6, 2020 |
| Entity Registrant Name | ULTRAGENYX PHARMACEUTICAL INC. |
| Entity Central Index Key | 0001515673 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-36276 |
| Entity Incorporation State Country Code | DE |
| Entity Tax Identification Number | 27-2546083 |
| Entity Address Address Line1 | 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 each class | Common Stock, $0.001 par value |
| Trading Symbol | RARE |
| Name of each exchange on which registered | NASDAQ |
