Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Total revenue	$ 4,110	$ 7,171
Type of Revenue [Extensible List]	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember
Operating expenses:
Research and development	$ 45,733	$ 34,605
General and administrative	11,012	14,644
Total operating expenses	56,745	49,249
Loss from operations	(52,635)	(42,078)
Other income, net	464	2,059
Net loss	(52,171)	(40,019)
Unrealized gain (loss) on marketable securities	(204)	2,640
Comprehensive loss	$ (52,375)	$ (37,379)
Net loss per share, basic and diluted	$ (0.66)	$ (0.53)
Shares used in computing net loss per share, basic and diluted	79,386,836	74,820,950