| Document And Entity Information | Jul. 29, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jul. 29, 2021 |
| Entity Registrant Name | Alector, Inc. |
| Entity Central Index Key | 0001653087 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-38792 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 82-2933343 |
| Entity Address, Address Line One | 131 Oyster Point Blvd. |
| Entity Address, Address Line Two | Suite 600 |
| Entity Address, City or Town | South San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94080 |
| City Area Code | (415) |
| Local Phone Number | 231-5660 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | ALEC |
| Security Exchange Name | NASDAQ |
