Cover Page - shares	9 Months Ended
	Sep. 30, 2021	Nov. 01, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Entity Registrant Name	Sierra Oncology, Inc.
Entity Central Index Key	0001290149
Trading Symbol	SRRA
Current Fiscal Year End Date	--12-31
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		15,055,040
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.001 par value
Entity File Number	001-37490
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-0138994
Entity Address, Address Line One	1820 Gateway Drive, 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
Document Quarterly Report	true
Document Transition Report	false