Document And Entity Information - shares
6 Months Ended
Jun. 30, 2019
Aug. 09, 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 PSNL  
Entity Registrant Name Personalis, Inc.  
Entity Central Index Key 0001527753  
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   31,128,674
Entity Current Reporting Status No  
Entity Shell Company false  
Entity File Number 001-38943  
Entity Tax Identification Number 275411038  
Entity Address, Address Line One 1330 O’Brien Drive  
Entity Address, City or Town Menlo Park  
Entity Address, State or Province California  
Entity Address, Postal Zip Code 94025  
City Area Code 650  
Local Phone Number 752-1300