| Document And Entity Information | Nov. 12, 2019 |
| Cover [Abstract] | |
| Entity Registrant Name | Allakos Inc. |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Central Index Key | 0001564824 |
| Document Period End Date | Nov. 12, 2019 |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| 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 |
| Entity Address, Address Line Two | 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 Each Class | Common Stock, par value $0.001 |
| Trading Symbol | ALLK |
| Name of Each Exchange on Which Registered | NASDAQ |
