Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Feb. 21, 2023	Jun. 30, 2022
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	FY
Trading Symbol	ALEC
Entity File Number	001-38792
Entity Tax Identification Number	82-2933343
Entity Public Float			$ 627.1
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
Document Transition Report	false
Document Annual Report	true
Entity Registrant Name	Alector, Inc.
Entity Central Index Key	0001653087
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		83,027,910
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock
Entity Interactive Data Current	Yes
Entity Well-known Seasoned Issuer	No
Entity Incorporation, State or Country Code	DE
Documents Incorporated by Reference	Portions of the registrant’s Definitive Proxy Statement relating to the registrant’s 2023 Annual Meeting of Stockholders are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days after the end of the registrant’s 2022 fiscal year ended December 31, 2022.
Auditor Name	Ernst & Young LLP
Auditor Location	San Mateo, California
Auditor Firm ID	42