Stock-Based Compensation Expense Related to Stock Options and RSUs (Detail) - Stock Options and Restricted Stock Units - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	$ 703	$ 664	$ 1,987	$ 1,916
Cost of revenue
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	22	24	63	71
Research and Development Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	125	130	397	361
Selling and Marketing Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	171	193	519	463
General and Administrative Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Expense	$ 385	$ 317	$ 1,008	$ 1,021