Balance Sheet Components - Schedule of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment	$ 225	$ 15
Accumulated depreciation and amortization	(25)	(2)
Property and equipment, net	200	13
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	11	8
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 214	$ 7