XML 23 R1.htm IDEA: XBRL DOCUMENT v3.20.4
Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Mar. 08, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2020    
Document Fiscal Year Focus 2020    
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   11,628,479  
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     $ 56.1
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 2021 Annual Meeting of Stockholders will be filed with the Commission within 120 days after the end of the Registrant’s 2020 fiscal year and is incorporated by reference into Part III of this Report.