| Document and Entity Information Document | Feb. 25, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 25, 2021 |
| Entity Registrant Name | Allogene Therapeutics, Inc. |
| Entity Central Index Key | 0001737287 |
| Amendment Flag | false |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38693 |
| Entity Tax Identification Number | 82-3562771 |
| Entity Address, Address Line One | 210 East Grand Avenue |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | 650 |
| Local Phone Number | 457-2700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value per share |
| Trading Symbol | ALLO |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
