Segment, Customer Concentration and Geographical Information - Company Revenue in Different Geographic Locations (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Total revenues	$ 5,732	$ 5,804	$ 5,862	$ 5,576	$ 7,084	$ 6,478	$ 7,459	$ 7,214	$ 22,974	$ 28,235	$ 39,507
Americas [Member]
Revenue from External Customer [Line Items]
Total revenues									22,579	27,618	39,008
EMEA [Member]
Revenue from External Customer [Line Items]
Total revenues									170	424	239
Asia Pacific [Member]
Revenue from External Customer [Line Items]
Total revenues									$ 225	$ 193	$ 260