Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Mar. 07, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
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		23,665,100
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]	Portions of the Registrant’s Definitive Proxy Statement (“Proxy Statement”) relating to the 2022 Annual Meeting of Stockholders will be filed with the Commission within 120 days after the end of the Registrant’s 2021 fiscal year and is incorporated by reference into Part III of this Report.
Auditor Firm ID	34
Auditor Name	Deloitte & Touche LLP
Auditor Location	Grand Rapids, Michigan