| Document and Entity Information | Apr. 12, 2022 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001290149 |
| Document Type | 8-K |
| Document Period End Date | Apr. 12, 2022 |
| Entity Registrant Name | SIERRA ONCOLOGY, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-37490 |
| Entity Tax Identification Number | 20-0138994 |
| Entity Address, Address Line One | 1820 Gateway Drive |
| Entity Address, Address Line Two | Suite 110 |
| Entity Address, City or Town | San Mateo |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94404 |
| City Area Code | (650) |
| Local Phone Number | 376-8679 |
| Written Communications | false |
| Soliciting Material | true |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.001 par value |
| Trading Symbol | SRRA |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
