Business Segment, Geographic, and Concentration Risk Information - Summary of Property and Equipment by Geographical Segment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Total property and equipment, net:
Total property and equipment, net	$ 9,651	$ 5,810
United States
Total property and equipment, net:
Total property and equipment, net	5,305	2,177
China
Total property and equipment, net:
Total property and equipment, net	$ 4,346	$ 3,633