Segment Information - Geographical Segment Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Total	$ 5,613	$ 6,091
Domestic [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total	1,451	1,501
International [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total	$ 4,162	$ 4,590