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.