| Cover Page - USD ($) $ in Millions | 12 Months Ended | ||
| Dec. 31, 2021 | Apr. 25, 2022 | Jun. 30, 2021 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Amendment Flag | true | ||
| Document Period End Date | Dec. 31, 2021 | ||
| Document Fiscal Year Focus | 2021 | ||
| Document Fiscal Period Focus | FY | ||
| Entity Registrant Name | Sierra Oncology, Inc. | ||
| Entity Central Index Key | 0001290149 | ||
| Trading Symbol | SRRA | ||
| Current Fiscal Year End Date | --12-31 | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Voluntary Filers | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Shell Company | false | ||
| Entity Filer Category | Non-accelerated Filer | ||
| Entity Emerging Growth Company | false | ||
| Entity Small Business | true | ||
| Entity Common Stock, Shares Outstanding | 24,419,349 | ||
| Entity Interactive Data Current | Yes | ||
| Security Exchange Name | NASDAQ | ||
| Title of 12(b) Security | Common Stock, $0.001 par value | ||
| Entity Address, Address Line One | 1820 Gateway Drive, Suite 110 | ||
| Entity Address, City or Town | San Mateo | ||
| Entity Address, State or Province | CA | ||
| Entity Public Float | $ 128.4 | ||
| Document Annual Report | true | ||
| Document Transition Report | false | ||
| Entity File Number | 001-37490 | ||
| Entity Tax Identification Number | 20-0138994 | ||
| Entity Address, Postal Zip Code | 94404 | ||
| City Area Code | 650 | ||
| Local Phone Number | 376-8679 | ||
| ICFR Auditor Attestation Flag | false | ||
| Entity Incorporation, State or Country Code | DE | ||
| Document Information [Text Block] | None. | ||
| Auditor Firm ID | 34 | ||
| Auditor Name | Deloitte & Touche LLP | ||
| Auditor Location | Grand Rapids, Michigan | ||
| Amendment Description | This Amendment No. 1 on Form 10-K/A (this Form 10-K/A) to the Annual Report on Form 10-K of Sierra Oncology, Inc., a Delaware corporation (referred to as “SRRA,” the “Company,” “we,” “us,” or “our”) for the fiscal year ended December 31, 2021, originally filed with the Securities and Exchange Commission (SEC) on March 10, 2022 (the Original 10-K), is being filed for the purpose of including the information required by Part III of Form 10-K. This information was previously omitted from the Original 10-K in reliance on General Instruction G(3) to Form 10-K, which permits the information in the above referenced items to be incorporated in the Form 10-K by reference from our definitive proxy statement if such proxy statement is filed no later than 120 days after our fiscal year-end. The reference on the cover page of the Original 10-K to the incorporation by reference to portions of our definitive proxy statement into Part III of the Original 10-K has been deleted. This Amendment No. 1 hereby amends and restates in their entirety the cover page and Items 10 through 14 of Part III of the Original 10-K. As required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended, in connection with this Form 10-K/A, our Chief Executive Officer and Chief Financial Officer are providing Rule 13a-14(a) certifications as included herein. We are amending Item 15 of Part IV solely to reflect the inclusion of these certifications. Except as described above, this Form 10K/A does not modify or update disclosure in, or exhibits to, the Original 10-K. Furthermore, this Form 10-K/A does not change any previously reported financial results, nor does it reflect events occurring after the filing date of the Original 10-K. Information not affected by this Form 10-K/A remains unchanged and reflects the disclosures made at the time the Original 10-K was filed. |
