| Document And Entity Information | Nov. 30, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 30, 2021 |
| Entity Registrant Name | Allakos Inc. |
| Entity Central Index Key | 0001564824 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-38582 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 45-4798831 |
| Entity Address, Address Line One | 975 Island Drive, 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 |
| Trading Symbol | ALLK |
| Security Exchange Name | NONE |
