Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Nov. 05, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Trading Symbol ALLK  
Entity Registrant Name Allakos Inc.  
Entity Central Index Key 0001564824  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   48,665,674
Entity Current Reporting Status Yes  
Entity Shell Company false  
Entity File Number 001-38582  
Entity Tax Identification Number 45-4798831  
Entity Address, Address Line One 975 Island Drive  
Entity Address, Address Line Two Suite 201  
Entity Address, City or Town Redwood City  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94065  
City Area Code 650  
Local Phone Number 597-5002  
Entity Interactive Data Current Yes  
'Title of 12(b) Security Common Stock, par value $0.001  
Security Exchange Name NASDAQ  
Document Quarterly Report true  
Document Transition Report false  
Entity Incorporation, State or Country Code DE