Segment and Geographic Information - Summary of Revenue by Geographic Area Based on the Customers' Location (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Entity Wide Revenue Major Customer [Line Items]
Total revenue	$ 80,962	$ 34,042	$ 22,237
United States
Entity Wide Revenue Major Customer [Line Items]
Total revenue	34,231	11,506	1,106
Reportable Geographical Components | United States
Entity Wide Revenue Major Customer [Line Items]
Total revenue	34,231	11,506	1,106
Reportable Geographical Components | Germany
Entity Wide Revenue Major Customer [Line Items]
Total revenue	13,727	0	0
Reportable Geographical Components | Rest of World
Entity Wide Revenue Major Customer [Line Items]
Total revenue	$ 33,004	$ 22,536	$ 21,131