Significant Customers and Concentrations of Risk - Summary of Revenues by Customer Location (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
Concentration Risk [Line Items]
Total revenues	$ 14,040	$ 15,052	$ 13,416	$ 12,007	$ 14,196	$ 13,479	$ 12,228	$ 12,319	$ 54,515	$ 52,222	$ 45,658
United States of America [Member]
Concentration Risk [Line Items]
Total revenues									48,924	45,531	39,229
Europe [Member]
Concentration Risk [Line Items]
Total revenues									5,061	6,117	6,026
Other [Member]
Concentration Risk [Line Items]
Total revenues									$ 530	$ 574	$ 403