Document And Entity Information - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Mar. 04, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Entity Registrant Name Allogene Therapeutics, Inc.    
Document Type 10-K    
Trading Symbol ALLO    
Current Fiscal Year End Date --12-31    
Entity Common Stock, Shares Outstanding   121,482,671  
Amendment Flag false    
Entity Central Index Key 0001737287    
Entity Filer Category Non-accelerated Filer    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Entity Emerging Growth Company true    
Entity Small Business false    
Entity Ex Transition Period false    
Entity Shell Company false    
Entity Current Reporting Status Yes    
Entity Public Float     $ 0
Entity Voluntary Filers No    
Entity Well-known Seasoned Issuer No