Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Total revenue	$ 5,904	$ 2,696	$ 16,245	$ 15,218
Type of Revenue [Extensible List]	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember	alec:CollaborationRevenueMember
Operating expenses:
Research and development	$ 43,819	$ 28,519	$ 112,486	$ 74,766
General and administrative	15,834	8,326	46,175	22,514
Total operating expenses	59,653	36,845	158,661	97,280
Loss from operations	(53,749)	(34,149)	(142,416)	(82,062)
Other income, net	1,045	2,411	4,367	7,204
Net loss	(52,704)	(31,738)	(138,049)	(74,858)
Unrealized gain (loss) on marketable securities	(752)	(215)	952	268
Comprehensive loss	$ (53,456)	$ (31,953)	$ (137,097)	$ (74,590)
Net loss per share, basic and diluted	$ (0.67)	$ (0.47)	$ (1.78)	$ (1.25)
Shares used in computing net loss per share, basic and diluted	78,771,930	67,572,452	77,340,896	59,663,773