Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 06, 2019
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
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	Accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	true
Entity Common Stock, Shares Outstanding		74,688,283
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock
Entity Address, State or Province	BC
Entity Address, Country	CA