Stock-Based Compensation Expense Related to Stock Options and RSUs (Detail) - Stock Options and Restricted Stock Units - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	$ 635	$ 591
Cost of revenue
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	19	24
Research and Development Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	135	115
Selling and Marketing Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	158	160
General and Administrative Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	$ 323	$ 292