Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Nov. 02, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
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	true
Entity Ex Transition Period	true
Entity Small Business	true
Entity Common Stock, Shares Outstanding		11,040,600
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	c/o 2150 – 885 West Georgia Street
Entity Address, City or Town	Vancouver
Entity Address, State or Province	BC
Entity Address, Country	CA
Entity Address, Postal Zip Code	V6C 3E8
City Area Code	604
Local Phone Number	558-6536
Document Quarterly Report	true
Document Transition Report	false