Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 31, 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  
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   43,185,253
Entity Current Reporting Status Yes  
Entity Shell Company false  
Entity File Number 001-38582  
Entity Tax Identification Number 454798831  
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 California  
Entity Address, Postal Zip Code 94065  
City Area Code 650  
Local Phone Number 597-5002