Segment Information and Information about Geographic Areas - Property and Equipment, Net By Geographic Area (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	$ 8,589	$ 8,088
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	7,182	7,063
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Property and equipment, net	$ 1,407	$ 1,025